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# sumDecompositionString -- produces a simplified diagonal representative of a Grothendieck Witt class

## Synopsis

• Usage:
sumDecompositionString(beta)
• Inputs:
• beta, , a symmetric bilinear form defined over a field $k$
• Outputs:
• , the decomposition as a sum of hyperbolic and rank one forms

## Description

Given a symmetric bilinear form beta over a field $k$, we return a simplified diagonal form of beta.

 i1 : M = matrix(RR,{{2.091,2.728,6.747},{2.728,7.329,6.257},{6.747,6.257,0.294}}); 3 3 o1 : Matrix RR <-- RR 53 53 i2 : beta = gwClass(M); i3 : sumDecompositionString(beta) o3 = 1H+ <1>

Over $\mathbb{R}$ there are only two square classes and a form is determined uniquely by its rank and signature [L05, II Proposition 3.2]. A form defined by the $3\times 3$ Gram matrix M above is isomorphic to the form $\langle 1,-1,1\rangle$.

 i4 : M = matrix(GF(13),{{9,1,7,4},{1,10,3,2},{7,3,6,7},{4,2,7,5}}); 4 4 o4 : Matrix (GF 13) <-- (GF 13) i5 : beta = gwClass(M); i6 : sumDecompositionString(beta) o6 = 1H+ <1>+ <-5>

Over $\mathbb{F}_{q}$ forms can similarly be diagonalized. In this case as $\langle 1,-1,1,-6 \rangle$.

Citations:

• [L05] T.Y. Lam, Introduction to quadratic forms over fields, American Mathematical Society, 2005.