Concept

Minor — where it appears

The determinant of a square submatrix, chosen by a set of rows and a set of columns. Minors are what an elimination's intermediate entries turn out to be when the arithmetic is kept exact, which is why a bound on determinants bounds the whole run before it starts.

Named by 3 essays across one field — each of them below, with the objects they name alongside it.

Also named here as sylvesters identity — the same set of essays touches all of them, so they are one junction rather than several.

1-2-6-7-474-26-12-5-8-524-394151435A, the matrix as given1-2-6-7-401840552700112217207003943011710017279153after 2 fraction-free steps3 × 3 minors25 divisions, all exactthe intermediates are minorswhich is why the divisions come out whole

Every intermediate is a minor

Fraction-free elimination divides by the previous pivot at every step and the division is always exact. Not usually, not for these entries — always, because the number being divided is a determinant with that pivot as a factor, which is a theorem and is checked here against the minors themselves.

exact · Fraction-free
10×10, ±6determinant, bits27Hadamard's bound36median slack10024680510152025303540elimination stepbitsHadamard, on the minorsthe intermediatesevery intermediate is a minor of the originalso a bound on the minors bounds the run

A bound on every intermediate at once

Fraction-free elimination's intermediates are minors of the original, which is a theorem about exactness. It is also a bound: Hadamard's inequality applies to every minor, so one inequality bounds the whole run before it starts. The bound on the k-th step is the one on (k+1)×(k+1) minors, not the one on the whole matrix — and on a 10×10 with entries in ±6 the difference is ten bits, with the run reaching 2.7 bits a step against the bound's 3.4.

exact · Fraction-free
20 matricespeak, every rule29the determinant's own length29area, smallest ÷ natural0.9602468051015202530elimination stepbits, longest entrythe determinant: 29 bitsswap only on a zerolargest pivotsmallest nonzero pivotthree orders, three sequences of minorsand one last entry they all arrive at

Three orders and one last entry

Over the integers there is no stability to pivot for, so a fraction-free elimination swaps rows only when the pivot is zero. Choosing a pivot for length instead does change the sequence of minors — the smallest-nonzero rule makes seven exchanges where the natural order makes none and keeps the profile two bits lower through the middle. It cannot change the peak. The last entry of the elimination is the determinant, and the determinant does not know what order it was computed in.

exact · Fraction-free

Named alongside it

The objects these essays reach for when they reach for this one.

Bit lengthDeterminantExact arithmeticFraction-free eliminationHadamard boundSylvesters identityExact ground truthFlop countGaussian eliminationPartial pivotingPermutation

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