# Question:some problems with maple

## Question:some problems with maple

Maple

i have a matrix named S. it has two variables with the names sigma1 and tau. i do some computations with these varibels to obtain a matrix named S_NEW. when i calculate S_NEW with parameters and then i substitute parameter tau with zero i got the answer a zero Matrix. but when i put tau=0 at the initiation of my code, i got another answer. what is the problem?

the second issue is related with the name of parametes in maple. when the varible has the name sigma1 (and tau=0), the matrix S_NEW is a matrix with the only component of (1,1). when i change the parameter name to sigma the matrix S_NEW is a matrix with the only component of (3,3). where is the problem?

 > restart:with(LinearAlgebra):
 > tau:=0;
 (1)
 > S:=Matrix(3,[[sigma1,tau,0],[tau,0,0],[0,0,0]]):B1:=Matrix(3):
 > A12,B12:=(Eigenvectors(S)):
 > V12:= Matrix(Normalize~([Column(B12,1..3)],Euclidean)):
 > S_NEW:=simplify(V12^+.S.V12) assuming real ;
 (2)
 > eval(S_NEW,tau=0)
 (3)
 >
 >
 >
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