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A product of matrixes that returns 0. instead of the correct result

Posted 10 years ago
Hi all. Today i bumped into this problem:
 h = 0.25;
 El = 43*10^3;
 Et = 8.9*10^3;
 Glt = 4.5*10^3;
 vlt = 0.27;
 n = 4;
 vtl =vlt*Et/El;
 Dloc = 1/(1 - vlt*vtl)*{{El, El*vtl, 0}, {El*vtl, Et,0}, {0, 0, (1 -vlt*vtl)*Glt}};

c1 = Cos[Pi/4];
s1 = Sin[Pi/4];
T1 = {{c1^2, s1^2, c1*s1}, {s1^2, c1^2, -c1*s1}, {-2*c1*s1, 2*c1*s1, c1^2 - s1^2}};

c2 = Cos[-Pi/4];
s2 = Sin[-Pi/4];
T2 = {{c2^2, s2^2, c2*s2}, {s2^2, c2^2, -c2*s2}, {-2*c2*s2, 2*c2*s2, c2^2 - s2^2}};

D1 = Transpose[T1].Dloc.T1;
D2 = Transpose[T2].Dloc.T2;
Dm = 1/n*(2*D1 + 2*D2)

beta= -(n*h)^2/(2*n)
Dmf = {{0, 0, beta*D2[[1, 3]]}, {0, 0, beta*D2[[2, 3]]}, {beta*D2[[1, 3]], beta*D2[[2, 3]], 0}}

A1 = Inverse[Dm]
B1 = -A1*Dmf
The fact is that Mathematica returns me a B1 matrix full of  "0." when it actually shouldn't.
How is it possible that some of the elements of A1 (numbers of the order of 0.0000x)  multiplied by some of the elements of Dmf (of the order of 10xx) return 0?
I tried with //N //MatrixForm everywhere and with zero results.
Looking forward to your help. Thanks.
POSTED BY: Andrea Gazzotti
3 Replies
But then the answer is fine, since for each of the 9 matrix elements either A1 or Dmf has a zero entry, which results in a product of 0.
POSTED BY: Ilian Gachevski
* is element-wise multiplication, perhaps you meant -A1.Dmf
POSTED BY: Ilian Gachevski
Posted 10 years ago
No, sorry, no error there. I need that kind of multiplication.
POSTED BY: Andrea Gazzotti
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