FAILURE MAP
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FA-14576 / Numerics / Open access

Rational mediant walk: mediant construction · case 01

The exact rational mediant walk result violates the stated contract at mediant construction.

Verified by executionVariant 1 · 8 checks per implementationDownload source bundle ↓JSON ↗

ROOT CAUSE

The mediant construction step uses [lo[0]+hi[1],lo[1]+hi[0]] instead of [lo[0]+hi[0],lo[1]+hi[1]].

THE FAILURE

The mediant construction step uses [lo[0]+hi[1],lo[1]+hi[0]] instead of [lo[0]+hi[0],lo[1]+hi[1]].

Unsuccessful approach: The partial repair [lo[0]+hi[0],max(lo[1],hi[1])] still violates the mediant construction invariant.

Case contract

Input path over L/R; return Stern-Brocot interval bounds and its mediant.

Why this case matters

Exact discrete arithmetic with observable algorithmic state; no floating point approximation is used.

1 / The failure

Exit 1
"""Failure Map reference implementation. Python standard library only."""
import json
import math
import itertools
from fractions import Fraction
N = 1
observations = []
def solve(x):
    lo=[0,1]
    hi=[1,0]
    for step in x:
     mid=[lo[0]+hi[1],lo[1]+hi[0]]
     if step=='L': hi=mid
     else: lo=mid
    return [lo,hi,[lo[0]+hi[0],lo[1]+hi[1]]]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[('L', [[0, 1], [1, 1], [1, 2]]), ('LL', [[0, 1], [1, 2], [1, 3]]), ('', [[0, 1], [1, 0], [1, 1]]), ('RRRRRR', [[6, 1], [1, 0], [7, 1]]), ('R', [[1, 1], [1, 0], [2, 1]]), ('LR', [[1, 2], [1, 1], [2, 3]]), ('RL', [[1, 1], [2, 1], [3, 2]]), ('RR', [[2, 1], [1, 0], [3, 1]])], [('R', [[1, 1], [1, 0], [2, 1]]), ('LLR', [[1, 3], [1, 2], [2, 5]]), ('', [[0, 1], [1, 0], [1, 1]]), ('RRRRRR', [[6, 1], [1, 0], [7, 1]]), ('LLRL', [[1, 3], [2, 5], [3, 8]]), ('LLRR', [[2, 5], [1, 2], [3, 7]]), ('LRLL', [[1, 2], [3, 5], [4, 7]]), ('LRLR', [[3, 5], [2, 3], [5, 8]])], [('LL', [[0, 1], [1, 2], [1, 3]]), ('RLL', [[1, 1], [3, 2], [4, 3]]), ('', [[0, 1], [1, 0], [1, 1]]), ('RRRRRR', [[6, 1], [1, 0], [7, 1]]), ('LLLRR', [[2, 7], [1, 3], [3, 10]]), ('LLRLL', [[1, 3], [3, 8], [4, 11]]), ('LLRLR', [[3, 8], [2, 5], [5, 13]]), ('LLRRL', [[2, 5], [3, 7], [5, 12]])], [('LR', [[1, 2], [1, 1], [2, 3]]), ('LLLR', [[1, 4], [1, 3], [2, 7]]), ('', [[0, 1], [1, 0], [1, 1]]), ('RRRRRR', [[6, 1], [1, 0], [7, 1]]), ('RLRLL', [[3, 2], [8, 5], [11, 7]]), ('RLRLR', [[8, 5], [5, 3], [13, 8]]), ('RLRRL', [[5, 3], [7, 4], [12, 7]]), ('RLRRR', [[7, 4], [2, 1], [9, 5]])], [('RL', [[1, 1], [2, 1], [3, 2]]), ('LRLL', [[1, 2], [3, 5], [4, 7]]), ('', [[0, 1], [1, 0], [1, 1]]), ('RRRRRR', [[6, 1], [1, 0], [7, 1]]), ('LLLRLR', [[3, 11], [2, 7], [5, 18]]), ('LLLRRL', [[2, 7], [3, 10], [5, 17]]), ('LLLRRR', [[3, 10], [1, 3], [4, 13]]), ('LLRLLL', [[1, 3], [4, 11], [5, 14]])]]
for i, (args, expected) in enumerate(fixtures[N-1]):
    check("explicit oracle %d" % i, solve(args), expected)
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)
Boundary fixtureActualExpectedOutcome
explicit oracle 0[[0, 1], [0, 2], [0, 3]][[0, 1], [1, 1], [1, 2]]Failed
explicit oracle 1[[0, 1], [2, 1], [2, 2]][[0, 1], [1, 2], [1, 3]]Failed
explicit oracle 2[[0, 1], [1, 0], [1, 1]][[0, 1], [1, 0], [1, 1]]Passed
explicit oracle 3[[0, 7], [1, 0], [1, 7]][[6, 1], [1, 0], [7, 1]]Failed
explicit oracle 4[[0, 2], [1, 0], [1, 2]][[1, 1], [1, 0], [2, 1]]Failed
explicit oracle 5[[2, 1], [0, 2], [2, 3]][[1, 2], [1, 1], [2, 3]]Failed
explicit oracle 6[[0, 2], [0, 3], [0, 5]][[1, 1], [2, 1], [3, 2]]Failed
explicit oracle 7[[0, 3], [1, 0], [1, 3]][[2, 1], [1, 0], [3, 1]]Failed

SHA-256 / 04c9ad8631da746025aeacc2494337e6e8c914fb467976aab2cb184cf1be6050

2 / The unsuccessful fix

Exit 1
"""Failure Map reference implementation. Python standard library only."""
import json
import math
import itertools
from fractions import Fraction
N = 1
observations = []
def solve(x):
    lo=[0,1]
    hi=[1,0]
    for step in x:
     mid=[lo[0]+hi[0],max(lo[1],hi[1])]
     if step=='L': hi=mid
     else: lo=mid
    return [lo,hi,[lo[0]+hi[0],lo[1]+hi[1]]]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[('L', [[0, 1], [1, 1], [1, 2]]), ('LL', [[0, 1], [1, 2], [1, 3]]), ('', [[0, 1], [1, 0], [1, 1]]), ('RRRRRR', [[6, 1], [1, 0], [7, 1]]), ('R', [[1, 1], [1, 0], [2, 1]]), ('LR', [[1, 2], [1, 1], [2, 3]]), ('RL', [[1, 1], [2, 1], [3, 2]]), ('RR', [[2, 1], [1, 0], [3, 1]])], [('R', [[1, 1], [1, 0], [2, 1]]), ('LLR', [[1, 3], [1, 2], [2, 5]]), ('', [[0, 1], [1, 0], [1, 1]]), ('RRRRRR', [[6, 1], [1, 0], [7, 1]]), ('LLRL', [[1, 3], [2, 5], [3, 8]]), ('LLRR', [[2, 5], [1, 2], [3, 7]]), ('LRLL', [[1, 2], [3, 5], [4, 7]]), ('LRLR', [[3, 5], [2, 3], [5, 8]])], [('LL', [[0, 1], [1, 2], [1, 3]]), ('RLL', [[1, 1], [3, 2], [4, 3]]), ('', [[0, 1], [1, 0], [1, 1]]), ('RRRRRR', [[6, 1], [1, 0], [7, 1]]), ('LLLRR', [[2, 7], [1, 3], [3, 10]]), ('LLRLL', [[1, 3], [3, 8], [4, 11]]), ('LLRLR', [[3, 8], [2, 5], [5, 13]]), ('LLRRL', [[2, 5], [3, 7], [5, 12]])], [('LR', [[1, 2], [1, 1], [2, 3]]), ('LLLR', [[1, 4], [1, 3], [2, 7]]), ('', [[0, 1], [1, 0], [1, 1]]), ('RRRRRR', [[6, 1], [1, 0], [7, 1]]), ('RLRLL', [[3, 2], [8, 5], [11, 7]]), ('RLRLR', [[8, 5], [5, 3], [13, 8]]), ('RLRRL', [[5, 3], [7, 4], [12, 7]]), ('RLRRR', [[7, 4], [2, 1], [9, 5]])], [('RL', [[1, 1], [2, 1], [3, 2]]), ('LRLL', [[1, 2], [3, 5], [4, 7]]), ('', [[0, 1], [1, 0], [1, 1]]), ('RRRRRR', [[6, 1], [1, 0], [7, 1]]), ('LLLRLR', [[3, 11], [2, 7], [5, 18]]), ('LLLRRL', [[2, 7], [3, 10], [5, 17]]), ('LLLRRR', [[3, 10], [1, 3], [4, 13]]), ('LLRLLL', [[1, 3], [4, 11], [5, 14]])]]
for i, (args, expected) in enumerate(fixtures[N-1]):
    check("explicit oracle %d" % i, solve(args), expected)
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)
Boundary fixtureActualExpectedOutcome
explicit oracle 0[[0, 1], [1, 1], [1, 2]][[0, 1], [1, 1], [1, 2]]Passed
explicit oracle 1[[0, 1], [1, 1], [1, 2]][[0, 1], [1, 2], [1, 3]]Failed
explicit oracle 2[[0, 1], [1, 0], [1, 1]][[0, 1], [1, 0], [1, 1]]Passed
explicit oracle 3[[6, 1], [1, 0], [7, 1]][[6, 1], [1, 0], [7, 1]]Passed
explicit oracle 4[[1, 1], [1, 0], [2, 1]][[1, 1], [1, 0], [2, 1]]Passed
explicit oracle 5[[1, 1], [1, 1], [2, 2]][[1, 2], [1, 1], [2, 3]]Failed
explicit oracle 6[[1, 1], [2, 1], [3, 2]][[1, 1], [2, 1], [3, 2]]Passed
explicit oracle 7[[2, 1], [1, 0], [3, 1]][[2, 1], [1, 0], [3, 1]]Passed

SHA-256 / 4c914d4627ea975f0634ea632f66d16eeb1e8d6b1b3627c9fb642b861b2f51cf

HELD IN THE MEMBER ARCHIVE

The verified repair and its recorded checks are member-only.

This mechanism has 8 recorded checks per implementation. The open-access tier publishes the failure and the unsuccessful fix; the repaired source that passes every check, and the observations that prove it, are available to members.

Every case sharing this mechanism uses the same contract and the same repair, so this one record is held back for all of them.

Member access is invitation-based. Sign in with your invited account to inspect the repair.

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Verification & scope

A deterministic bounded teaching model. Inputs are restricted to the explicit contract; this is not a production algebra library. This reproducer isolates one failure mechanism. Results cover the supplied fixtures. Variants within a family share a test contract and should remain grouped when constructing evaluation splits. Related mechanisms with a shared evaluation_group must also remain together; these controlled models are not independent production incidents.

Observations recorded using Python 3.12.14 at 2026-09-29T14:39:18.389414+00:00.

Case digest / a607f1f8f32d559259ee743b602a1d536b1c3bc11ccaa3ec5429a383d997bd48