# With qc for j 1 r vj 1 n aj 1 n j vj

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Unformatted text preview: (j : n 1 : n) = row.house(C(j : n j : n) v(j : n)) end { This can be done in O(n2r) operations. Direct computation of Q requires O(n3) operations 10 Rotations De nition 2: A rotation G(i j 2 1 6. 6. 60 6 G(i j ) = 6 .. 6 6 60 6. 4. 0 ) 2 <n n has the form 0 .. 0 .. .. .. c s ;s c .. 0 .. 0 i c = cos 3 0 .. 7 7 07 i 7 .. 7 7 7 07 j .. 7 5 1 (4a) j s = sin { The transformation is called a (4b) plane rotation Givens rotation Some Properties of Givens Rotations { G(i j ) is orthogonal { G(i j ) is a rank-2 modi cation of I { The multiplication G(i j )T x is a counterclockwise rotation of x in the i j plane 11 Rotations If x 2 <n then y = G(i j )T x 8 < cxi ; sxj if k = i yk = : sxi + cxj if k = j xk otherwise If we want yj = 0 then c = q xi x2 + x2 i j s = q ;xj x2 + x2 i j (5a) (5b) Calculation of a Givens rotation function c, s] = givens(a, b) % givens: Compute the Givens transformation c, s such % that ca - sb = r and sa + cb = 0. if b = 0 c=1 s=0 elseif j b j > j a j tau = -a/b s = 1/ sqrt(1 + tau^2) c = s*tau else tau = -b/a c = 1/ sqrt(1 + tau^2) s = c*tau end 12 Rotations Function for a pre-Givens rotation { The multiplication only a ects rows i and j { Regard A as a 2 q matrix and compute c B = s ;s A c B is 2 q { Overwrite B with A function A = row.rot(A, c, s) % row.rot: Compute the rotation G(i j )A where % A 2 <2 q . The rotation is...
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## This document was uploaded on 03/16/2014 for the course CSCI 6800 at Rensselaer Polytechnic Institute.

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