# Multiplication of incompatible matrices

1. May 9, 2012

### attaran

Dear members,

I have a rather silly question.
As we all know only the compatible matrices can be multiplied. My derivation of some Finite Element formulation has, however, led me to the multiplication of two incompatible matrices.

I was wondering if we could make these incompatible matrices, compatible by some factorization techniques. Has anyone ever encountered such a problem?

I appreciate if someone could help me in this respect.

Regards,

-- Hamid Attaran

2. May 9, 2012

### chiro

Hey attaran and welcome to the forums.

Can you provide more information? What dimensions are the matrices? Are the matrices special types? (Like diagonal, triangular, singular, non-singular, have specific determinant, eigenvalues, eigenvectors and so on)

3. May 9, 2012

### attaran

Some are diagonal and some are not.
For one case I have reached from:
[A]8x2.2x2.[C]2x4.[D]4x1.[E]2x8
to:
[]8x1.[]2x8

which obviously are not compatible for matrix multiplication.

These are populated as follows:

A = [A11 0
0 A22
...
0 A82]

B = [B11 0
0 B22 ]

C = [C11 C12 C13 C14
C21 C22 C23 C24]

D = [D11
D21
D31
D41]

E= [E11 0 E13 0 ... E17 0
E21 0 E23 0 ... E27 0]

I hope I have provided enough information.

regards,
-- Hamid

4. May 9, 2012

### DonAntonio

Well, either (1) you define a new multiplication between non-compatible matrices, or (2) you embed all your matrices into

one single set of square matrices, perhaps by adding zeros to their rows or columns, or (3) you plainly cannot multiply those matrices among them.

I wonder why you think, or feel, that you must multiply those matrices...?

DonAntonio

5. May 9, 2012

### HallsofIvy

Staff Emeritus
The point is that if your method arrives at the multiplication of two "incompatible" matrices, then this cannot be ordinary multiplication of matrices. How you would alter the usual definition of multiplication of matrices would depend upon what you want this multiplication to mean. And that can only be determined by the precise problem you are dealing with.

Last edited: May 9, 2012
6. May 9, 2012

### AlephZero

Most likely you have got confused about something.

Computer programs for the finite element method often store data in 2-d arrays which are NOT "matrices" in the mathematical sense. For example the 6 independent compoents of a symmetric 3x3 tensor (for eaxmple stress and strain) are often stored in a 6x1 vector. Or 21 independent constants for an arbitrary anisotropic material might be stored in a symmetric 6x6 matrix, when it is really a 3x3x3x3 fourth-order tensor with a large number of symmetry relations between the 81 terms.

7. May 10, 2012