# SVD -- singular value decomposition of a matrix

## Synopsis

• Usage:
(S,U,Vt) = SVD M
• Inputs:
• M, , over RR or CC, of size m by n
• Optional inputs:
• DivideConquer => ..., default value false, Use the lapack divide and conquer SVD algorithm
• Outputs:
• S, , the list of singular values
• U, , an orthogonal (unitary) matrix of size m by m
• Vt, , an orthogonal (unitary) matrix of size n by n

## Description

If Sigma is the diagonal m by n matrix whose (i,i) entry is the i-th element of S, then M = U Sigma Vt. This is the singular value decomposition of M. The entries of S are (up to roundoff error) the eigenvalues of the Hermitian matrix M * (conjugate transpose M)

M may also be a MutableMatrix in which case the returned values U and Vt are also mutable matrices.

If M is over CC, then U and Vt are unitary matrices over CC. If M is over RR, U and Vt are orthogonal over RR.

 i1 : printingPrecision=2; i2 : M = map(RR^3, RR^5, (i,j) -> (i+1)^j * 1.0) o2 = | 1 1 1 1 1 | | 1 2 4 8 16 | | 1 3 9 27 81 | 3 5 o2 : Matrix RR <--- RR 53 53 i3 : (S,U,V) = SVD(M) o3 = ({88 }, | -.016 -.4 -.92 |, | -.014 -.038 -.11 -.32 -.94 |) {3.9} | -.21 -.89 .4 | | -.28 -.41 -.57 -.59 .29 | {.8 } | -.98 .2 -.068 | | -.74 -.41 .066 .51 -.15 | | .084 .33 -.81 .47 -.081 | | -.61 .74 .077 -.27 .062 | o3 : Sequence i4 : M' = (transpose U) * M * (transpose V) o4 = | 88 1.8e-14 1.7e-14 -3.8e-15 -1e-15 | | -2.7e-15 3.9 -6.6e-15 -1e-15 -2.2e-15 | | -8.3e-16 1.2e-14 .8 3.1e-16 6.7e-16 | 3 5 o4 : Matrix RR <--- RR 53 53
We can clean the small entries from the result above with clean.
 i5 : e = 1e-10; i6 : clean_e M' o6 = | 88 0 0 0 0 | | 0 3.9 0 0 0 | | 0 0 .8 0 0 | 3 5 o6 : Matrix RR <--- RR 53 53 i7 : clean_e norm (1 - U * transpose U) o7 = 0 o7 : RR (of precision 53)
Alternatively, if the only issue is display of the matrix, we may set the printing accuracy.
 i8 : printingAccuracy = 2 o8 = 2 i9 : M' o9 = | 88 0 0 -0 -0 | | -0 3.9 -0 -0 -0 | | -0 0 .8 0 0 | 3 5 o9 : Matrix RR <--- RR 53 53
Now let's try the divide and conquer algorithm and compare answers.
 i10 : (S', U', V') = SVD(M, DivideConquer => true) o10 = ({88 }, | -.02 -.4 -.92 |, | -.01 -.04 -.11 -.32 -.94 |) {3.9} | -.21 -.89 .4 | | -.28 -.41 -.57 -.59 .29 | {.8 } | -.98 .2 -.07 | | -.74 -.41 .07 .51 -.15 | | .08 .33 -.81 .47 -.08 | | -.61 .74 .08 -.27 .06 | o10 : Sequence i11 : norm \ ({S', U', V'}-{S, U, V}) o11 = {0, 0, 0} o11 : List
The SVD routine calls on the SVD algorithms in the lapack library.