Getting Nan after using expm

Question

raha ahmadi 2021-6-1

评论： raha ahmadi 2021-6-1

采纳的回答： Jan

Hi H dont know why I get Nan in situatin A but not in B.

Thanks in advance

% situation A
r=[1e5,0,0;
    0,-2e5,0;
    1,1,-5e5]
R=expm(r);
%% situation B
g=[-1e5,0,0;
    0,-2e5,0;
    1,1,-5e5]
G=expm(g);

Answer 1

Jan 2021-6-1

See the help text of expm:

Although it is not computed this way, if A has a full set of eigenvectors V with corresponding eigenvalues D then

% [V,D] = EIG(A) and expm(A) = V*diag(exp(diag(D)))/V
r=[1e5,0,0;  0,-2e5,0;  1,1,-5e5];
exp(diag(D))
%  [0; Inf; 0]

This Inf is the cause of the NaN output.

raha ahmadi 2021-6-1

I really appreciate for your quick response. In fact I expect Inf instead of NaN

Wish you all the best