FS0018

four_square_descent_nonzero_square

Alpha v34 independently verified · alpha_closed; checked-use authorized; not Stable

The square of a nonzero natural multiplier remains nonzero constructively.

Current library: Alpha v34, 4,223 checked-use theorems; Stable remains 432. Historical first admissions, original proof editions, and non-admitted aliases are preserved. Exact original first-admission records.

Exact expanded first-order arithmetic statement

forall k. ~(k = 0) -> ~(k * k = 0)

Constructive proof overview

Generated structural guide

The square of a nonzero natural multiplier remains nonzero constructively.

The unchanged tactic script uses 1 declared prerequisite and contains 13 exact native proof lines.

dependency-curried kernel-checked theorem body; Alpha enrollment and checked-use authority follow separately sealed release evidence; Stable membership remains unchanged

Historical empty-context replay experiment only; that experiment persisted no certificate and granted no release authority. Current checked use follows separately sealed, independently verified proof bundles; there is no Stable promotion.

Proof neighborhood

Direct dependencies

mul_eq_zero Stable theorem; checked-use authorized

Direct dependents

Formal native tactic body

Dependencies are introduced as named hypotheses before line 1. Local theorem links identify exact declared prerequisites. This exact body belongs to a complete independently kernel-checked constructive proof bundle and has Alpha checked-use authority; it does not imply Stable membership.

Read the argument

Proof checkpoints

13 script commands · 5 reading checkpoints · 1 local claims

This is a reading aid, not a new proof or a proof-tree certificate. Checkpoint groups are consecutive commands, not inferred branch boundaries. Every step links to the preserved script.

01Fix variables and assumptionsL1–3

Work with arbitrary variables or the premises of the current implication.

  1. L1
    intro k
  2. L2
    intro hnonzero
  3. L3
    intro hproduct
02Use earlier factsL4–5

Instantiate or apply named facts and discharge the corresponding proof obligations.

  1. L4
    specialize mul_eq_zero k
  2. L5
    specialize mul_eq_zero k
03Establish hcasesL6–8

Establish this local claim before using it. It is not an additional assumption. The following proof commands apply mul eq zero.

  1. L6
    have hcases : k = 0 \/ k = 0
  2. L7
    apply mul_eq_zero
  3. L8
    exact hproduct
04Separate the logical casesL9–9

Follow the explicit conjunction, disjunction, witness, or contradiction step recorded below.

  1. L9
    cases hcases
05Use earlier factsL10–13

Instantiate or apply named facts and discharge the corresponding proof obligations.

  1. L10
    apply hnonzero
  2. L11
    exact hcases_left
  3. L12
    apply hnonzero
  4. L13
    exact hcases_right

Library-wide reading audit

Original exact command ledger · 13 lines
  1. 0001intro k
  2. 0002intro hnonzero
  3. 0003intro hproduct
  4. 0004specialize mul_eq_zero k
  5. 0005specialize mul_eq_zero k
  6. 0006have hcases : k = 0 \/ k = 0
  7. 0007apply mul_eq_zero
  8. 0008exact hproduct
  9. 0009cases hcases
  10. 0010apply hnonzero
  11. 0011exact hcases_left
  12. 0012apply hnonzero
  13. 0013exact hcases_right