# `compact_arith`: searching for a small PA certificate A proof can be short on the blackboard and enormous after elaboration. That is not a soundness failure: every node of the enormous result may be correct. It is a lesson about the difference between three objects that are easy to conflate: 1. the **tactic program** a person writes; 2. the **proof tree** constructed by the untrusted engine; and 3. the judgment checked by the trusted kernel. This chapter develops `compact_arith`, a deliberately PA-specific arithmetic tactic. Its job is not to enlarge what Peano Lab can prove. Its job is to search for a smaller ordinary certificate for one rigid equality, using the direction in which PA's recursive equations are actually stated. The tactic remains untrusted, the kernel language remains unchanged, and “smallest found” never silently becomes “mathematically minimal.” ## The experiment that forced the question Consider the familiar parity theorem $$ \forall n\;\exists x\; n(n+1)=2x. $$ The historical pre-M18 readable replay first proves the stronger recurrence-normal statement $$ \forall n\;\exists x\; n^2+n=2x $$ and then transports it to the displayed theorem. Its source is only eighteen proof tactics, with two generic `ring` calls doing the arithmetic: ```text pa prove forall n. exists x. n * (n + 1) = 2 * x have strong : forall n. exists x. n * n + n = 2 * x induction n exists 0 norm_num cases IH exists x + S n trans ((n * n + n) + S n) + S n ring rewrite IH_witness ring intro n specialize strong n suffices normalize : n * (n + 1) = n * n + n rewrite normalize exact strong rewrite PA4 rewrite PA3 apply PA6 qed ``` Yet the finalized result has **30,030 structural proof-tree nodes**. Measuring the partial certificate after each interesting line locates the growth: | Point in the replay | Proof-tree nodes | |---|---:| | after `trans`, before the first `ring` | 35 | | after the first `ring` | 18,651 | | after rewriting by the induction hypothesis | 18,654 | | after the second `ring` | 30,016 | | after all source tactics, before local-cut compilation | 30,037 | | finalized ordinary kernel certificate | 30,030 | The two `ring` calls are doing honest work. The generic normalizer derives associativity, commutativity, distribution, and coefficient calculations by instantiating checked semiring-law certificates. Those derivations are then cut-normalized into the final tree. A high-level law name is not a one-node oracle in the kernel. Nor do `have` and `suffices` add sharing. They are convenient schedules for local cuts. Before checking, their engine-only nodes are compiled away. If a local fact is used repeatedly, its proof may consequently occur repeatedly in the tree. In this example the proof of `strong` is used only once, so local-cut duplication is not the main source of growth: generic `ring` elaboration is. This gives the first rule of the experiment: > Source-line count, rendered-character count, proof-tree count, and mathematical difficulty are > different measurements. M18 keeps the same human-visible mathematics but replaces those two generic algebra calls and the closed base calculation with the PA-specific tactic. The current replay is retained in [`artifacts/triangular-even-readable.pa`](https://github.com/nasqret/vietnam2026/blob/peano-lab/artifacts/triangular-even-readable.pa): ```text pa prove forall n. exists x. n * (n + 1) = 2 * x have strong : forall n. exists x. n * n + n = 2 * x induction n exists 0 compact_arith cases IH exists x + S n compact_arith [IH_witness] intro n specialize strong n suffices normalize : n * (n + 1) = n * n + n rewrite normalize exact strong compact_arith qed ``` Notice what did **not** disappear: the stronger claim, induction, both witness choices, the explicit permission to use `IH_witness`, and the final bridge. Compact arithmetic elaboration shortens the certificate construction; it does not conceal the mathematical proof plan. These thirteen proof tactics finalize to the same 180-node, depth-34 ordinary tree retained by the hand-authored certificate artifact; the canonical certificate bytes are identical. ## Discovery: normalize the recurrence, not every step The 30,030-node result prompted a sequence of checked experiments. A direct induction with a more careful successor witness produced 343 nodes. Replacing generic library derivations with smaller special-purpose arithmetic lemmas brought that route to 252 nodes. The decisive improvement was not a cleverer printer or a trusted shortcut. It was a better induction statement. PA defines operations by recursion on the **second** argument: $$ \begin{aligned} a+0 &= a &&\text{(PA3)},\\ a+S b &= S(a+b) &&\text{(PA4)},\\ a\cdot0 &= 0 &&\text{(PA5)},\\ a\cdot S b &= a\cdot b+a &&\text{(PA6)}. \end{aligned} $$ The surface term $n(n+1)$ therefore hides exactly the recurrence that PA6 wants to expose. Proving $n^2+n=2x$ inside the induction makes the induction hypothesis syntactically useful without renormalizing the original product at every successor step. After eliminating $$ n^2+n=2x, $$ choose the next witness $x+S n$. The remaining equality is organized as $$ (S n)^2+S n = (n^2+n)+S n+S n = 2x+S n+S n = 2(x+S n). $$ Only after the universal induction proof is complete do we prove $$ n(n+1)=n^2+n $$ and use one whole-proposition equality substitution to transport $$ \exists x\;(n^2+n=2x) \quad\text{to}\quad \exists x\;(n(n+1)=2x). $$ Transporting the existential as a whole is important. Opening it, recovering its witness, and rebuilding it would add logical scaffolding and would repeat a conversion that is independent of the witness. The reproducible constructor in [`scripts/minimize_parity_certificate.py`](https://github.com/nasqret/vietnam2026/blob/peano-lab/scripts/minimize_parity_certificate.py) builds a cut-normal **180-node**, depth-34 certificate. Its canonical rendering is retained as [`artifacts/triangular-even-180.certificate.txt`](https://github.com/nasqret/vietnam2026/blob/peano-lab/artifacts/triangular-even-180.certificate.txt). The independent kernel accepts it for the original theorem and rejects it for the nearby mutated claim $n(n+1)=2x+1$. The arithmetic subderivations explain where those nodes go: | Specialized derivation | Role | Tree nodes when measured on its stated instance | |---|---|---:| | `add_succ` | $S a+b=S(a+b)$ | 20 | | `swap_special` | move two successor additions in the exact required shape | 51 | | `mul_succ_left_special` | $S a\cdot n=a\cdot n+n$ | 75 | | `arithmetic_finish_special` | $(2w+j)+j=2(w+j)$ at the needed instance | 65 | These headline figures must not be added: some derivations occur inside others and the complete tree has its own surrounding logical nodes. The stronger universal theorem has 165 nodes; the final transport adds fifteen, giving 180. The regression suite pins that accounting more precisely. The closed base equality costs 9 nodes and its existential introduction adds one. The successor equality selected with `IH_witness` costs 149 nodes; existential introduction, elimination of the induction-hypothesis existential, and the induction-step binders add five. With the `Ind` node this makes the stronger universal theorem $10+(149+5)+1=165$. The final multiplication-by-`+1` bridge costs 11. Orienting it with `EqSym`, specializing `strong` with `ForallElim`, transporting the existential with `EqSubst`, and reintroducing the outer variable with `ForallIntro` add four, so the complete tree is $165+15=180$ at depth 34. ## The surface contract `compact_arith` deliberately does less than the hand-authored 180-node constructor. Version 1 closes one **rigid equality goal**. It does not invent an induction invariant, choose an existential witness, introduce variables, or secretly scan every hypothesis. The learner remains responsible for the mathematical shape of the proof. The two intended forms are: ```text compact_arith compact_arith [h, <- k] ``` With no bracketed list, the tactic uses only PA's defining equations and its fixed, checked recurrence templates. A bracketed list makes exactly those named equality hypotheses available, in the written order. The selected certificate may use a subset, but can never consult an unlisted equation. `h` offers its equation from left to right; `<- k` offers `k` from right to left. There is no wildcard, implicit context mining, or “use whatever works” mode. Consequently the pedagogical parity proof still has to say, visibly: - which stronger proposition to establish; - that induction is on `n`; - that the base witness is `0`; - that the successor witness is `x + S n`; - where the induction hypothesis may be used; and - how the recurrence-normal statement is transported back to the requested statement. `compact_arith` replaces only the bulky arithmetic equality derivations inside that structure. A pure `compact_arith?` preview reports the selected plan, equations actually used, expanded proof nodes, proof depth, annotation nodes, and synthesis work without changing goals, history, holes, trace steps, or allocator state. Preview is an inspection command, not a weaker acceptance path: running `compact_arith` must synthesize and check the certificate again. The following calls are outside the version-1 contract: ```text # not equality goals compact_arith # when the target is an existential or implication # no unresolved witness guessing compact_arith # when either equality side still contains a flexible metavariable # no hidden assumptions compact_arith [*] ``` Unsupported shapes, malformed lists, unknown or non-equality hypotheses, exhausted bounds, and different arithmetic normal forms are ordinary transactional tactic failures. None may publish a partial candidate. ## What certificate language may it use? There is no `CompactArith` node in the trusted language. The synthesizer targets the constructors already defined in `kernel/proofs.py`: | Kernel constructor | Arithmetic meaning | |---|---| | `Axiom("PA3")` ... `Axiom("PA6")` | the four recursive equations above | | `ForallElim` | instantiate a universally quantified PA equation or proved template | | `EqRefl`, `EqSym`, `EqTrans` | reflexivity, orientation, and equality paths | | `CongS`, `CongAdd`, `CongMul` | lift equalities through term constructors | | `EqSubst` | Leibniz transport, including replacement of several occurrences at once | | `ForallIntro`, `ImpIntro`, `Hyp`, `Ind` | construct the bounded recurrence templates themselves | The tactic needs no classical `DNE`, no new arithmetic axiom, and no engine-only local-cut node in the proof fragment it publishes. Logic constructors such as `ExistsIntro` remain the work of the surrounding human-written proof. A named helper is not trusted merely because the engine calls it a lemma. The fully quantified addition-successor template is paired with an ordinary certificate and checked once from the empty proof context. The parameter-specialized induction instances for offset swapping, successor-left multiplication, and doubling are likewise checked with no hypotheses before their final induction- index elimination. The small `+1`, `+2`, and multiplication-by-`+1` bridges are direct PA derivations rather than induction templates. Every selected focused result is cut-normalized and checked again. The kernel never resolves a helper name. ## A typed synthesis layer The safest implementation pattern is already present in `ring.py`: carry the claimed endpoints beside every proof fragment. Conceptually, the internal value is ```python EqualityCertificate(left, right, proof, cost) ``` Smart constructors enforce local contracts: - symmetry swaps the recorded endpoints; - transitivity requires the first right endpoint to equal the second left endpoint; - congruence constructs the corresponding compound endpoints; - a PA instance records the exact terms substituted for its quantifiers; and - equality substitution records both its one-hole formula motive and the direction of transport. These wrappers are untrusted bookkeeping. They make engine bugs easier to locate, but only the independent checker turns the underlying `proof` into evidence. The synthesis pipeline is: 1. **Validate the request.** Require a rigid equality, parse the optional ordered hypothesis list, reject unresolved metavariables, and scan the term AST under explicit limits. 2. **Seed exact edges.** Instantiate PA3--PA6, reflexivity, explicitly selected hypotheses, and the small recurrence templates at terms already present in the goal. 3. **Explore bounded alternatives.** The phase-1 seeded planner memoizes a finite candidate grammar: useful orientations, congruence positions, transitivity paths, equality-motive substitutions that may replace several term occurrences, and recurrence instances derived from goal subterms. It is not a general Dijkstra search, e-graph, or invariant synthesizer. 4. **Keep the cheapest exact endpoint proof found in that grammar.** Memoization may discard a more expensive derivation of the same syntactic equality. Ties use a documented structural key so browser and native runs agree. Candidates also retain their generation ordinal, so two equal-cost selected hypotheses respect the user's written order instead of being reordered by a set or dictionary traversal. 5. **Normalize administrative cuts.** Existing capture-avoiding reduction expands checked helper applications to the ordinary kernel tree. 6. **Measure the expanded candidate.** A one-node reference to a large helper is not charged as one if finalization will inline it. 7. **Check before commit.** Call the independent kernel on the exact focused context, candidate, and target. Only then replace the focused hole and append one history/trace transaction. 8. **Check again at QED.** Session finalization still compiles the complete proof and checks the session owner's original theorem in its original logic mode. This double check is intentional. The focused check prevents a bad arithmetic candidate from entering a live state; the final check prevents any tactic-layer or state-routing defect from changing the theorem that receives QED. ### Reading the implementation The complete untrusted engine is [`engine/compact_arith.py`](https://github.com/nasqret/vietnam2026/blob/peano-lab/peano-lab/py/peano_lab/engine/compact_arith.py). `CompactArithLimits` is the resource contract; `_EqualityProof` and `_Candidate` carry exact endpoints; `_Planner` performs bounded memoized selection; `prove_compact_equation` constructs and checks one focused fragment; and `compact_arith_checked` performs the immutable proof-state transaction. The parser, named-hypothesis resolution, preview, trace, replay, and tactical routing are in [`ui/prove.py`](https://github.com/nasqret/vietnam2026/blob/peano-lab/peano-lab/py/peano_lab/ui/prove.py). The tests are meant to be read beside that code. The [`engine tests`](https://github.com/nasqret/vietnam2026/blob/peano-lab/peano-lab/py/tests/test_compact_arith.py) pin endpoint composition, exact seed costs, deterministic selection, template checking, malformed inputs, and every resource boundary. The [`surface tests`](https://github.com/nasqret/vietnam2026/blob/peano-lab/peano-lab/py/tests/test_compact_arith_surface.py) pin preview purity, explicit hypotheses, trace/history/undo behavior, binders, and tacticals. The [`180-node replay`](https://github.com/nasqret/vietnam2026/blob/peano-lab/peano-lab/py/tests/test_readable_parity_artifact.py) then checks the complete original theorem, exact depth, and canonical certificate bytes. ## Recurrence templates, not a hidden decision procedure The useful search space is finite because the implementation fixes it, not because PA is easy. Version 1 seeds a small family of schema shapes suggested by PA's right-recursive equations: $$ \begin{aligned} a+1 &= S a,\\ a+2 &= S(Sa),\\ a(b+1) &= ab+a,\\ S a+b &= S(a+b),\\ S a\cdot b &= a\cdot b+b,\\ (A+j)+S a &= (A+a)+S j,\\ (2w+j)+j &= 2(w+j). \end{aligned} $$ The first three bridges cost 6, 10, and 11 proof nodes respectively. In particular, the 11-node multiplication-by-add-one bridge proves the final displayed normalization directly from a six-node successor bridge and PA6, avoiding a trip through the general polynomial normalizer. The four genuine recurrence identities after the three direct bridges are justified by `Ind` certificates. Parameters are shifted beneath the induction binder by the existing De Bruijn substitution utilities. The search instantiates a schema only at terms drawn from the focused equality and explicitly named hypotheses; it does not enumerate arbitrary formulas. The zero-offset form of the final doubling recurrence is a typical proof-engineering choice. Its base case reduces immediately with PA3 and PA5. The recorded hand experiment compared it with an algebraically equivalent successor-offset template, which looked closer to the goal but produced a larger certificate. Version 1 retains the 65-node zero-offset winner as its single doubling seed; that historical comparison is not performed again on every tactic call. This is not `omega`, nonlinear hypothesis solving, or a decision procedure for PA. It does not claim completeness even for all polynomial identities: `ring` remains the broad commutative- semiring normalizer. `compact_arith` trades breadth for certificate shape in a small, documented family of recurrence equalities. ## Cost means expanded proof-tree cost Peano Lab's existing metric is $$ |p| = 1 + \sum_{q\text{ a direct proof child of }p}|q|. $$ It counts occurrences of `Proof` constructors. It does not count term AST nodes or formula annotations stored in `EqSubst` and `Ind`. It also treats the certificate as a tree: if the same Python proof object appears twice, both occurrences are counted. Canonical text likewise prints both occurrences. This metric is useful because it describes the work visible to the current structural checker, but it is not the only reasonable metric. A richer future planner could retain a small Pareto frontier over at least: - expanded proof-tree nodes; - maximum proof depth; - term/formula annotation size or canonical rendered bytes; and - synthesis work and elapsed time. Version 1 instead memoizes one winner per exact endpoint pair and assumption-permission mode, using the deterministic lexicographic key `(expanded nodes, proof depth, annotation nodes, generation ordinal)`. The extra mode prevents an internal recurrence prefix from silently consuming a named hypothesis. Work units and elapsed time are stopping bounds, not optimization objectives. The public result must name which measure it reports. “120 unique subtrees” is not the same claim as “120 expanded nodes.” Hash-consing can reduce engine memory, but it does not reduce the current tree metric. A serialized DAG with references would require a new acyclicity- and context-aware kernel design—particularly because `Hyp` and binders make sharing scope-sensitive—and is outside this tactic. ## What may honestly be called minimal? The 180-node artifact is a **checked upper bound**: it proves that a certificate of size 180 exists. It is the smallest result found in the recorded experiment. It is not a proof that no 179-node certificate exists. The distinction is unusually important here because the node metric ignores the sizes of terms and motives. There are infinitely many possible annotations at a fixed constructor count, so “try all proofs with fewer nodes” is not automatically a finite computation. A global minimality claim would need both: 1. a precisely fixed, finite candidate language and cost model; and 2. an exhaustive search or a formally verified lower-bound argument for that language. `compact_arith` may make the narrower statement “cheapest among the candidates generated by template set T under bounds B,” provided it really enumerates that finite set and charges the post-expansion tree. It may report “matches the current 180-node record” if a complete surrounding proof actually does. It must not print “optimal certificate” as a synonym for “best route this heuristic happened to visit.” For comparison, changing the cost to canonical serialized bytes would make the set below each fixed cost finite in principle. Exhaustive search would still be fantastically expensive. No general tactic for arbitrary PA statements can promise to find a proof—or a smallest proof—and terminate on every input. ## Bounds and transactional failure Certificate minimization is search, and search needs visible stopping rules. The implementation therefore fixes limits for at least: - input term nodes and depth; - number and size of explicitly selected hypotheses; - recurrence-schema instantiations; - candidate equality endpoints and paths; - work units and wall-clock time; - generated proof nodes and depth; and - complete partial-proof nodes and depth after insertion. The version-1 defaults admit at most 256 aggregate input-term nodes at depth 64, 16 explicitly selected equalities, 64 seed/template instances, 512 memo/search states, 512 generated candidates, 100,000 term/formula annotation nodes at depth 256, 20,000 work units, a generated fragment of 10,000 proof nodes at depth 256, a complete partial certificate of 100,000 proof nodes at depth 256, and five seconds. Annotation accounting matters because the primary `proof_size` metric deliberately omits the terms and motives stored inside proof nodes. Limits are checked before expensive construction and throughout the search. Crossing one raises a typed tactic limit; it does not mean the equality is false, unprovable, or lacks a smaller proof. The immutable input `ProofState`, history, substitutions, and holes remain exactly unchanged. Likewise, a failed `compact_arith?` preview has no state effect. The browser's Stop action remains the hard interruption boundary because Python runs inside a disposable worker; restarting that worker discards the in-memory session rather than publishing an unfinished candidate. ## Tests are part of the theorem prover The central positive regression is not merely “the tactic returned success.” It should establish all of the following: - the readable parity proof supplies the stronger invariant, witnesses, and explicit IH use; - every `compact_arith` equality fragment checks in its exact local context; - the finalized complete certificate checks from the empty context against the original theorem; - its expanded node/depth metrics are deterministic and meet the recorded bound; and - the same certificate fails against a nearby mutated target. Across the existing kernel and arithmetic suites, adversarial tests mutate a PA axiom instance, equality orientation, witness, induction motive, substitution motive, and target. M18 adds exact- endpoint, orientation, and nearby-target attacks. Capture regressions run template use beneath extra universal, existential, and implication binders. Transaction tests cover malformed lists, unknown hypotheses, non-equation hypotheses, unresolved metavariables, different endpoints, exhausted budgets, host recursion failure, and a candidate rejected by the independent checker. Repeated runs must choose byte-identical plans. Preview must leave state, history, holes, metavariable allocation, and JSONL traces untouched. Running the tactic adds exactly one ordinary transaction, and one `undo` restores the exact earlier state. A static import test continues to prove that the kernel imports neither the new engine module nor any UI code. The most valuable test is still the ordinary last line: ```text qed ``` It submits the complete expanded certificate—not the cost report, recurrence plan, preview, or tactic's claim of success—to the unchanged checker with the original formula. At the M18 close, the focused suite has 46 passing tests and the full Peano suite has 744; the sibling Lambda suite remains green at 360 tests plus 36 subtests. The warning-as-error book build, 193-link/170-command executable-prose gate, 61-note/356-link connected vault, 1,692-session source- bound corpus, application and vendor manifests, Node interaction harnesses, and exact local stage are also green. Build `2026-07-28c`, application `a-953fa3777cd4`, is deployed to the staging channel from commit `98ee0dd`; the public HTML and all 41 application files match the local staged checksums. No in-app browser was attached, so this report does not claim a live Pyodide click-through. Production remains on M13 because the host still omits the required cache headers. ## What students should take away The 30,030-to-180 experiment is not an argument against readable tactics. Readability helped reveal the right invariant. Nor is it an argument that the kernel should trust a faster arithmetic oracle. It shows that proof engineering has at least two creative levels: - choose a proposition and witness whose recurrence matches the axioms; and - choose a certificate construction that avoids paying for general algebra when a specialized derivation is enough. `compact_arith` automates part of the second level while leaving the first visible. That boundary is pedagogically deliberate. A student can inspect why the step works, compare its certificate with `ring`, alter a recurrence template, and see the independent checker accept or reject the result. The lesson is not that 180 is a magical number. It is that optimization may change how evidence is built, but never who is allowed to validate it. Continue with {doc}`The deliberate limits ` for the boundary between bounded automation and general PA, or return to {doc}`Checked arithmetic automation ` to compare the contracts of `simp`, `norm_num`, `ring`, and `auto`.