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name: solve-modular-arithmetic locale: caveman-ultra source_locale: en source_commit: 82c77053 fence_basis_commit: 82c77053 translator: "Julius Brussee homage — caveman" translation_date: "2026-05-03" description: > Solve mod arithmetic probs: congruences, systems via CRT, mod inverses, Euler's theorem. Manual + computational. Use → linear congruences, mod inverses, large mod exponentiation, simultaneous congruences (CRT), cyclic groups + discrete log contexts. license: MIT allowed-tools: Read Bash metadata: author: Philipp Thoss version: "1.0" domain: number-theory complexity: intermediate language: multi tags: number-theory, modular-arithmetic, congruences, crt, euler

Solve Modular Arithmetic

Parse congruence systems → extended Euclidean for inverses → CRT for systems → Euler for exponentiation. Verify all by substitution.

Use When

• Single linear congruence ax = b (mod m)
• System of simultaneous congruences (CRT)
• Mod inverse a^{-1} (mod m)
• Large mod exp a^k (mod m)
• Order of element in Z/mZ
• Cyclic groups, primitive roots, discrete log

In

• Required: Congruence(s) | mod eqn
• Optional: Show extended Euclidean steps explicit?
• Optional: Apply Euler | Fermat?
• Optional: Find primitive roots | element orders?
• Optional: Output (step-by-step, compact, proof)

Do

Step 1: Parse System / Eqn

Extract math structure.

1. ID type:

   • Single linear: ax = b (mod m)
   • System: x = a1 (mod m1), x = a2 (mod m2), ...
   • Mod exp: a^k (mod m)
   • Mod inverse: find a^{-1} (mod m)

2. Normalize: Reduce coefs mod respective moduli. a, b, m non-neg, m > 0.

3. Record parsed problem in standard notation.

Got: Clearly parsed + normalized problem, all values reduced.

If err: Ambiguous notation ("solve 3x + 5 = 2 mod 7" → equation or 5=2 mod 7?) → clarify w/ user. Default: mod applies to entire eqn.

Step 2: Solve Single Congruence

ax = b (mod m) via extended Euclidean.

1. g = gcd(a, m) via Euclidean:

   • Repeated div: m = q1·a + r1, a = q2·r1 + r2, ... until rem = 0
   • Last non-zero rem = gcd(a, m)

2. Solvability: solution iff g | b. Else no solution. Stop.

3. Reduce: ÷ g → (a/g)x = (b/g) (mod m/g). Now gcd(a/g, m/g) = 1.

4. Find inverse of a/g mod m/g via extended Euclidean:

   • Back-sub through Euclidean → 1 = (a/g)·s + (m/g)·t
   • Coef s (reduced mod m/g) = inverse

5. Particular sol: x0 = s·(b/g) mod (m/g).

6. General sol: x = x0 + (m/g)·k for k = 0, 1, ..., g-1 → all g incongruent solutions mod m.

Extended Euclidean ex (find 17^{-1} mod 43):

43 = 2*17 + 9
17 = 1*9  + 8
 9 = 1*8  + 1
 8 = 8*1  + 0

Back-substitute:
1 = 9 - 1*8
  = 9 - 1*(17 - 1*9) = 2*9 - 17
  = 2*(43 - 2*17) - 17 = 2*43 - 5*17

So 17*(-5) = 1 (mod 43), i.e., 17^{-1} = -5 = 38 (mod 43).

Got: Complete solution set, or proof no solution exists.

If err: Back-sub wrong result → verify each div step. Most common err: sign mistake in back-sub. Check: a · inverse mod m should = 1.

Step 3: System via CRT

Solve x = a1 (mod m1), x = a2 (mod m2), ..., x = ak (mod mk).

1. Pairwise coprime check: every pair (mi, mj), gcd(mi, mj) = 1.

   • All coprime → CRT applies directly
   • Some not coprime → check compat: each non-coprime pair, ai = aj (mod gcd(mi, mj)). Compat → reduce via lcm. Incompat → no sol.

2. M = m1 · m2 · ... · mk (product of all).

3. Each i, Mi = M/mi (product except mi).

4. Each i, yi = Mi^{-1} (mod mi) via extended Euclidean (Step 2).

5. Solution: x = sum(ai · Mi · yi for i = 1..k) mod M.

6. State: x = [val] (mod M). Unique sol mod M.

Common totients ref:

n	phi(n)	n	phi(n)	n	phi(n)
2	1	10	4	20	8
3	2	11	10	24	8
4	2	12	4	25	20
5	4	13	12	30	8
6	2	14	6	36	12
7	6	15	8	48	16
8	4	16	8	60	16
9	6	18	6	100	40

Got: Unique sol mod M, or proof of incompat.

If err: CRT result fails verification → check Step 4 inverse calcs. Common: computing Mi^{-1} mod M instead of mod mi. Each inverse is mod individual mi, not product.

Step 4: Apply Euler / Fermat

Eval mod exponentiations | simplify w/ Euler.

1. Euler: gcd(a, m) = 1 → a^{phi(m)} = 1 (mod m).

   • phi(m): if m = p1^e1 · p2^e2 · ... · pk^ek, phi(m) = m · product((1 - 1/pi) per prime pi | m).

2. Fermat little (special): p prime, gcd(a, p) = 1 → a^{p-1} = 1 (mod p).

3. Reduce exp: compute a^k (mod m):

   • r = k mod phi(m)
   • a^k = a^r (mod m)

4. a^r (mod m) via repeated squaring (binary exp):

   • r in binary: r = b_n · 2^n + ... + b_1 · 2 + b_0
   • result = 1
   • Each bit MSB → LSB: result = result² mod m; bit = 1 → result = result · a mod m

5. gcd(a, m) > 1: Euler doesn't apply. Factor m + CRT to combine results from prime power moduli, lifting exponent | direct compute.

Got: a^k (mod m) value via exp reduction + repeated squaring.

If err: gcd(a, m) > 1 + result wrong → don't apply Euler. Compute direct | factor m into coprime parts where some coprime to a, solve mod each, recombine via CRT.

Step 5: Verify by Substitution

Plug solution back.

1. Single congruence: a · x mod m = b?

2. CRT systems: each x = ai (mod mi), x mod mi = ai?

3. Mod exponentiation: verify w/ 2nd method (direct for small | independent repeated squaring impl).

4. Document explicit:

Solution: x = 23
Check 1: 23 mod 3 = 2 = a1. Correct.
Check 2: 23 mod 5 = 3 = a2. Correct.
Check 3: 23 mod 7 = 2 = a3. Correct.
All congruences satisfied.

Got: All eqns verified w/ explicit computation.

If err: Verify fails → trace back. Common: arith mistakes in extended Euclidean, wrong sign in back-sub, forget reduce mod M in final CRT step.

Check

• Type ID'd (single, system, exp, inverse)
• All coefs reduced mod respective moduli
• ax = b (mod m): gcd(a, m) | b checked before solving
• Extended Euclidean back-sub verified: a · inverse mod m = 1
• CRT: pairwise coprime verified before
• CRT non-coprime: compat checked
• Euler applied only when gcd(a, m) = 1
• phi(m) from prime factorization, not guessed
• Repeated squaring uses mod reduction every step (no overflow)
• Every sol verified by substitution

Traps

• CRT w/o coprime check: Standard formula needs pairwise coprime. Non-coprime → wrong answer not error. Check gcd(mi, mj) = 1 first.
• Wrong inverse: Mi^{-1} mod mi (individual), NOT mod M (product). Most common CRT impl err.
• Euler when gcd(a, m) > 1: a^{phi(m)} = 1 (mod m) needs gcd = 1. Else theorem doesn't apply, result wrong.
• Sign errs in extended Euclidean back-sub: Track signs each step. Final inverse may be neg → reduce mod m for positive rep.
• Overflow in mod exp: Even repeated squaring → intermediate products overflow. Reduce mod m after every mult, not just end.
• Forget multiple solutions: ax = b (mod m), g = gcd(a, m) > 1, g | b → exactly g incongruent solutions mod m, not just one.

→

• analyze-prime-numbers — prime fact needed for phi(m) + verify coprime
• explore-diophantine-equations — linear Diophantine ax + by = c equiv to linear congruence ax = c (mod b)
• prove-geometric-theorem — mod arith in constructibility proofs (which regular n-gons are constructible)