Re: Eigenvalues works very slow

*To*: mathgroup at smc.vnet.net*Subject*: [mg128491] Re: Eigenvalues works very slow*From*: David Bailey <dave at removedbailey.co.uk>*Date*: Thu, 25 Oct 2012 01:40:32 -0400 (EDT)*Delivered-to*: l-mathgroup@mail-archive0.wolfram.com*Delivered-to*: l-mathgroup@wolfram.com*Delivered-to*: mathgroup-newout@smc.vnet.net*Delivered-to*: mathgroup-newsend@smc.vnet.net*References*: <k685fc$m70$1@smc.vnet.net>

On 24/10/2012 08:30, jure lapajne wrote: > Hello, > I'm trying to calculate eigenvalues of different sized matrices (from 10x10 to 1000x1000) using mathematica's built-in function - Eigenvalues. For smaller matrices it works ok, but for larger matrices it's just too slow. I tried writing the code in another system and it finds eigenvalues very quickly (in a second or two at most) even for big matrices. I'm not sure whether am I doing something wrong or is the speed difference really this big. > My code (you only need to change d to change the size of matrix): > d = 25; > q = Table[Table[1/2*Sqrt[i + j + 1]*KroneckerDelta[Abs[i - j], 1], {j, 0, d-1}],{i, 0, d - 1}]; > h0 = Table[Table[(i + 1/2)*KroneckerDelta[i, j], {j, 0, d - 1}], {i, 0,d-1}]; > lambda = 1/2; > q4 = lambda*q.q.q.q; > N[Eigenvalues[q4 + h0]] > > Thanks for help. > You are calculating eigenvalues of an integer matrix - so you will get a result in terms of complicated fractions, that you then reduce to floating point numbers using N ! Change the last line to Eigenvalues[N[q4 + h0]] David Bailey http://www.dbaileyconsultancy.co.uk