Question 8985: Both Parenthesizations of a Three-Input GCD

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Original question and givens
Definition and scope
Step-by-step derivation
Reverse checks
Result
This question review and source binding

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R0016-08985 · q-20261013-advance-20261018-r03-gcd-coprime-0085

Original question and givens

Natural inputs a=21, b=36, c=57, with gcd(0,0)=0. Give (a, b, c, gcd(gcd(a,b),c), gcd(a,gcd(b,c)), are the results equal?).

G001 — Natural inputs a=21, b=36, c=57, with gcd(0,0)=0. Give (a, b, c, gcd(gcd(a,b),c), gcd(a,gcd(b,c)), are the results equal?).

Definition and scope

Use the original question domain, assumptions and cited MKO definitions. This item has second-context mathematical and bilingual review, with the model, answer and every arithmetic step independently recomputed from the original parameters; the source question remains a candidate.

mko-gcd-coprime

Step-by-step derivation

S001 · implies

Before

natural triple=[21, 36, 57];gcd(0,0)=0

After

compute both complete parenthesizations

Conditions: none

Compute both actual Euclidean routes rather than repeating the associativity conclusion.Inputs may include0.

Step identifiers and dependencies

Rule: gcd_two_routes_scope · Uses: G001

S002 · implies

Before

left inner gcd(a,b) Euclid row0:divide natural 21 by positive 36

After

21=0·36+21

Conditions: none

A positive divisor gives a unique quotient/remainder with0≤r<divisor.The full equality audits both values without division by zero.

Step identifiers and dependencies

Rule: natural_euclidean_division_witness · Uses: G001, S001

S003 · implies

Before

phase=left inner gcd(a,b);pair=[21, 36];quotient=0;remainder=21

After

next pair=[36, 21];0<=remainder<divisor

Conditions: none

From x=q*y+r,a number divides both x,y exactly when it divides both y,r.The remainder is strictly smaller than the positive divisor,so the second argument eventually becomes0.

Step identifiers and dependencies

Rule: gcd_common_divisor_invariance · Uses: G001, S002

S004 · implies

Before

left inner gcd(a,b) Euclid row1:divide natural 36 by positive 21

After

36=1·21+15

Conditions: none

A positive divisor gives a unique quotient/remainder with0≤r<divisor.The full equality audits both values without division by zero.

Step identifiers and dependencies

Rule: natural_euclidean_division_witness · Uses: G001, S003

S005 · implies

Before

phase=left inner gcd(a,b);pair=[36, 21];quotient=1;remainder=15

After

next pair=[21, 15];0<=remainder<divisor

Conditions: none

From x=q*y+r,a number divides both x,y exactly when it divides both y,r.The remainder is strictly smaller than the positive divisor,so the second argument eventually becomes0.

Step identifiers and dependencies

Rule: gcd_common_divisor_invariance · Uses: G001, S004

S006 · implies

Before

left inner gcd(a,b) Euclid row2:divide natural 21 by positive 15

After

21=1·15+6

Conditions: none

A positive divisor gives a unique quotient/remainder with0≤r<divisor.The full equality audits both values without division by zero.

Step identifiers and dependencies

Rule: natural_euclidean_division_witness · Uses: G001, S005

S007 · implies

Before

phase=left inner gcd(a,b);pair=[21, 15];quotient=1;remainder=6

After

next pair=[15, 6];0<=remainder<divisor

Conditions: none

From x=q*y+r,a number divides both x,y exactly when it divides both y,r.The remainder is strictly smaller than the positive divisor,so the second argument eventually becomes0.

Step identifiers and dependencies

Rule: gcd_common_divisor_invariance · Uses: G001, S006

S008 · implies

Before

left inner gcd(a,b) Euclid row3:divide natural 15 by positive 6

After

15=2·6+3

Conditions: none

A positive divisor gives a unique quotient/remainder with0≤r<divisor.The full equality audits both values without division by zero.

Step identifiers and dependencies

Rule: natural_euclidean_division_witness · Uses: G001, S007

S009 · implies

Before

phase=left inner gcd(a,b);pair=[15, 6];quotient=2;remainder=3

After

next pair=[6, 3];0<=remainder<divisor

Conditions: none

From x=q*y+r,a number divides both x,y exactly when it divides both y,r.The remainder is strictly smaller than the positive divisor,so the second argument eventually becomes0.

Step identifiers and dependencies

Rule: gcd_common_divisor_invariance · Uses: G001, S008

S010 · implies

Before

left inner gcd(a,b) Euclid row4:divide natural 6 by positive 3

After

6=2·3+0

Conditions: none

A positive divisor gives a unique quotient/remainder with0≤r<divisor.The full equality audits both values without division by zero.

Step identifiers and dependencies

Rule: natural_euclidean_division_witness · Uses: G001, S009

S011 · implies

Before

phase=left inner gcd(a,b);pair=[6, 3];quotient=2;remainder=0

After

next pair=[3, 0];0<=remainder<divisor

Conditions: none

From x=q*y+r,a number divides both x,y exactly when it divides both y,r.The remainder is strictly smaller than the positive divisor,so the second argument eventually becomes0.

Step identifiers and dependencies

Rule: gcd_common_divisor_invariance · Uses: G001, S010

S012 · implies

Before

phase=left inner gcd(a,b);terminal pair=[3, 0]

After

gcd=3

Conditions: none

At termination gcd(x,0)=x,including the natural convention gcd(0,0)=0.Never take a remainder modulo0,and do not infer that both inputs are prime.

Step identifiers and dependencies

Rule: gcd_terminal_zero_convention · Uses: G001, S011

S013 · implies

Before

left outer gcd(ab,c) Euclid row0:divide natural 3 by positive 57

After

3=0·57+3

Conditions: none

A positive divisor gives a unique quotient/remainder with0≤r<divisor.The full equality audits both values without division by zero.

Step identifiers and dependencies

Rule: natural_euclidean_division_witness · Uses: G001, S012

S014 · implies

Before

phase=left outer gcd(ab,c);pair=[3, 57];quotient=0;remainder=3

After

next pair=[57, 3];0<=remainder<divisor

Conditions: none

From x=q*y+r,a number divides both x,y exactly when it divides both y,r.The remainder is strictly smaller than the positive divisor,so the second argument eventually becomes0.

Step identifiers and dependencies

Rule: gcd_common_divisor_invariance · Uses: G001, S013

S015 · implies

Before

left outer gcd(ab,c) Euclid row1:divide natural 57 by positive 3

After

57=19·3+0

Conditions: none

A positive divisor gives a unique quotient/remainder with0≤r<divisor.The full equality audits both values without division by zero.

Step identifiers and dependencies

Rule: natural_euclidean_division_witness · Uses: G001, S014

S016 · implies

Before

phase=left outer gcd(ab,c);pair=[57, 3];quotient=19;remainder=0

After

next pair=[3, 0];0<=remainder<divisor

Conditions: none

From x=q*y+r,a number divides both x,y exactly when it divides both y,r.The remainder is strictly smaller than the positive divisor,so the second argument eventually becomes0.

Step identifiers and dependencies

Rule: gcd_common_divisor_invariance · Uses: G001, S015

S017 · implies

Before

phase=left outer gcd(ab,c);terminal pair=[3, 0]

After

gcd=3

Conditions: none

At termination gcd(x,0)=x,including the natural convention gcd(0,0)=0.Never take a remainder modulo0,and do not infer that both inputs are prime.

Step identifiers and dependencies

Rule: gcd_terminal_zero_convention · Uses: G001, S016

S018 · implies

Before

right inner gcd(b,c) Euclid row0:divide natural 36 by positive 57

After

36=0·57+36

Conditions: none

A positive divisor gives a unique quotient/remainder with0≤r<divisor.The full equality audits both values without division by zero.

Step identifiers and dependencies

Rule: natural_euclidean_division_witness · Uses: G001, S017

S019 · implies

Before

phase=right inner gcd(b,c);pair=[36, 57];quotient=0;remainder=36

After

next pair=[57, 36];0<=remainder<divisor

Conditions: none

From x=q*y+r,a number divides both x,y exactly when it divides both y,r.The remainder is strictly smaller than the positive divisor,so the second argument eventually becomes0.

Step identifiers and dependencies

Rule: gcd_common_divisor_invariance · Uses: G001, S018

S020 · implies

Before

right inner gcd(b,c) Euclid row1:divide natural 57 by positive 36

After

57=1·36+21

Conditions: none

A positive divisor gives a unique quotient/remainder with0≤r<divisor.The full equality audits both values without division by zero.

Step identifiers and dependencies

Rule: natural_euclidean_division_witness · Uses: G001, S019

S021 · implies

Before

phase=right inner gcd(b,c);pair=[57, 36];quotient=1;remainder=21

After

next pair=[36, 21];0<=remainder<divisor

Conditions: none

From x=q*y+r,a number divides both x,y exactly when it divides both y,r.The remainder is strictly smaller than the positive divisor,so the second argument eventually becomes0.

Step identifiers and dependencies

Rule: gcd_common_divisor_invariance · Uses: G001, S020

S022 · implies

Before

right inner gcd(b,c) Euclid row2:divide natural 36 by positive 21

After

36=1·21+15

Conditions: none

A positive divisor gives a unique quotient/remainder with0≤r<divisor.The full equality audits both values without division by zero.

Step identifiers and dependencies

Rule: natural_euclidean_division_witness · Uses: G001, S021

S023 · implies

Before

phase=right inner gcd(b,c);pair=[36, 21];quotient=1;remainder=15

After

next pair=[21, 15];0<=remainder<divisor

Conditions: none

From x=q*y+r,a number divides both x,y exactly when it divides both y,r.The remainder is strictly smaller than the positive divisor,so the second argument eventually becomes0.

Step identifiers and dependencies

Rule: gcd_common_divisor_invariance · Uses: G001, S022

S024 · implies

Before

right inner gcd(b,c) Euclid row3:divide natural 21 by positive 15

After

21=1·15+6

Conditions: none

A positive divisor gives a unique quotient/remainder with0≤r<divisor.The full equality audits both values without division by zero.

Step identifiers and dependencies

Rule: natural_euclidean_division_witness · Uses: G001, S023

S025 · implies

Before

phase=right inner gcd(b,c);pair=[21, 15];quotient=1;remainder=6

After

next pair=[15, 6];0<=remainder<divisor

Conditions: none

From x=q*y+r,a number divides both x,y exactly when it divides both y,r.The remainder is strictly smaller than the positive divisor,so the second argument eventually becomes0.

Step identifiers and dependencies

Rule: gcd_common_divisor_invariance · Uses: G001, S024

S026 · implies

Before

right inner gcd(b,c) Euclid row4:divide natural 15 by positive 6

After

15=2·6+3

Conditions: none

A positive divisor gives a unique quotient/remainder with0≤r<divisor.The full equality audits both values without division by zero.

Step identifiers and dependencies

Rule: natural_euclidean_division_witness · Uses: G001, S025

S027 · implies

Before

phase=right inner gcd(b,c);pair=[15, 6];quotient=2;remainder=3

After

next pair=[6, 3];0<=remainder<divisor

Conditions: none

From x=q*y+r,a number divides both x,y exactly when it divides both y,r.The remainder is strictly smaller than the positive divisor,so the second argument eventually becomes0.

Step identifiers and dependencies

Rule: gcd_common_divisor_invariance · Uses: G001, S026

S028 · implies

Before

right inner gcd(b,c) Euclid row5:divide natural 6 by positive 3

After

6=2·3+0

Conditions: none

A positive divisor gives a unique quotient/remainder with0≤r<divisor.The full equality audits both values without division by zero.

Step identifiers and dependencies

Rule: natural_euclidean_division_witness · Uses: G001, S027

S029 · implies

Before

phase=right inner gcd(b,c);pair=[6, 3];quotient=2;remainder=0

After

next pair=[3, 0];0<=remainder<divisor

Conditions: none

From x=q*y+r,a number divides both x,y exactly when it divides both y,r.The remainder is strictly smaller than the positive divisor,so the second argument eventually becomes0.

Step identifiers and dependencies

Rule: gcd_common_divisor_invariance · Uses: G001, S028

S030 · implies

Before

phase=right inner gcd(b,c);terminal pair=[3, 0]

After

gcd=3

Conditions: none

At termination gcd(x,0)=x,including the natural convention gcd(0,0)=0.Never take a remainder modulo0,and do not infer that both inputs are prime.

Step identifiers and dependencies

Rule: gcd_terminal_zero_convention · Uses: G001, S029

S031 · implies

Before

right outer gcd(a,bc) Euclid row0:divide natural 21 by positive 3

After

21=7·3+0

Conditions: none

A positive divisor gives a unique quotient/remainder with0≤r<divisor.The full equality audits both values without division by zero.

Step identifiers and dependencies

Rule: natural_euclidean_division_witness · Uses: G001, S030

S032 · implies

Before

phase=right outer gcd(a,bc);pair=[21, 3];quotient=7;remainder=0

After

next pair=[3, 0];0<=remainder<divisor

Conditions: none

From x=q*y+r,a number divides both x,y exactly when it divides both y,r.The remainder is strictly smaller than the positive divisor,so the second argument eventually becomes0.

Step identifiers and dependencies

Rule: gcd_common_divisor_invariance · Uses: G001, S031

S033 · implies

Before

phase=right outer gcd(a,bc);terminal pair=[3, 0]

After

gcd=3

Conditions: none

At termination gcd(x,0)=x,including the natural convention gcd(0,0)=0.Never take a remainder modulo0,and do not infer that both inputs are prime.

Step identifiers and dependencies

Rule: gcd_terminal_zero_convention · Uses: G001, S032

S034 · implies

Before

gcd(a,b)=3;gcd(b,c)=3

After

left=3;right=3;equal=Yes

Conditions: none

Feed each inner result into its own outer route and compare.The general reason is equality of the three-number common-divisor sets,but this instance retains the actual quotient/remainder witnesses.

Step identifiers and dependencies

Rule: gcd_parenthesizations_actual_comparison · Uses: G001, S033

Reverse checks

S034 · premise_and_result_audit — Check against the original data and this step's premises; a one-way implication is not an equivalence.
S033 · premise_and_result_audit — Check against the original data and this step's premises; a one-way implication is not an equivalence.
S032 · premise_and_result_audit — Check against the original data and this step's premises; a one-way implication is not an equivalence.
S031 · premise_and_result_audit — Check against the original data and this step's premises; a one-way implication is not an equivalence.
S030 · premise_and_result_audit — Check against the original data and this step's premises; a one-way implication is not an equivalence.
S029 · premise_and_result_audit — Check against the original data and this step's premises; a one-way implication is not an equivalence.
S028 · premise_and_result_audit — Check against the original data and this step's premises; a one-way implication is not an equivalence.
S027 · premise_and_result_audit — Check against the original data and this step's premises; a one-way implication is not an equivalence.
S026 · premise_and_result_audit — Check against the original data and this step's premises; a one-way implication is not an equivalence.
S025 · premise_and_result_audit — Check against the original data and this step's premises; a one-way implication is not an equivalence.
S024 · premise_and_result_audit — Check against the original data and this step's premises; a one-way implication is not an equivalence.
S023 · premise_and_result_audit — Check against the original data and this step's premises; a one-way implication is not an equivalence.
S022 · premise_and_result_audit — Check against the original data and this step's premises; a one-way implication is not an equivalence.
S021 · premise_and_result_audit — Check against the original data and this step's premises; a one-way implication is not an equivalence.
S020 · premise_and_result_audit — Check against the original data and this step's premises; a one-way implication is not an equivalence.
S019 · premise_and_result_audit — Check against the original data and this step's premises; a one-way implication is not an equivalence.
S018 · premise_and_result_audit — Check against the original data and this step's premises; a one-way implication is not an equivalence.
S017 · premise_and_result_audit — Check against the original data and this step's premises; a one-way implication is not an equivalence.
S016 · premise_and_result_audit — Check against the original data and this step's premises; a one-way implication is not an equivalence.
S015 · premise_and_result_audit — Check against the original data and this step's premises; a one-way implication is not an equivalence.
S014 · premise_and_result_audit — Check against the original data and this step's premises; a one-way implication is not an equivalence.
S013 · premise_and_result_audit — Check against the original data and this step's premises; a one-way implication is not an equivalence.
S012 · premise_and_result_audit — Check against the original data and this step's premises; a one-way implication is not an equivalence.
S011 · premise_and_result_audit — Check against the original data and this step's premises; a one-way implication is not an equivalence.
S010 · premise_and_result_audit — Check against the original data and this step's premises; a one-way implication is not an equivalence.
S009 · premise_and_result_audit — Check against the original data and this step's premises; a one-way implication is not an equivalence.
S008 · premise_and_result_audit — Check against the original data and this step's premises; a one-way implication is not an equivalence.
S007 · premise_and_result_audit — Check against the original data and this step's premises; a one-way implication is not an equivalence.
S006 · premise_and_result_audit — Check against the original data and this step's premises; a one-way implication is not an equivalence.
S005 · premise_and_result_audit — Check against the original data and this step's premises; a one-way implication is not an equivalence.
S004 · premise_and_result_audit — Check against the original data and this step's premises; a one-way implication is not an equivalence.
S003 · premise_and_result_audit — Check against the original data and this step's premises; a one-way implication is not an equivalence.
S002 · premise_and_result_audit — Check against the original data and this step's premises; a one-way implication is not an equivalence.
S001 · premise_and_result_audit — Check against the original data and this step's premises; a one-way implication is not an equivalence.

Result

(21, 36, 57, 3, 3, Yes)

The full substitutions and premise checks give the original answer; source identity,format and status are unchanged.

This question review and source binding

Review date: 2026-10-26

Original questions, answers, step expressions, numeric ASTs and historical records are retained. Source candidate labels remain historical and are not overwritten as formal proof.

Source hashes and review receipts

input commit: 0f7be61e2c22317a4075561dc822c87d881a50db
original question SHA-256: 973938429101914f4e0fd609e9965877ecdddb238afd7c6a069c2d84f3cbc87d
candidate record SHA-256: bd3ad89cc58dd68eeb64da5d44873ac1f54cc3d815ca6260a828d4cb817830a6
semantic review receipt SHA-256: 00fcddb10a9fb0f56697308976a52e68e2537472bd8e68ecbe96d3c4553f4ad2
question-first witness SHA-256: 198e04380b2d32aef2375b07ca0df1f8afa454d1184993173fad8a3c9187a3d1
Lean: NOT_RUN · human review: NOT_PERFORMED

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