Basics for recursive functions

Question

I am new to Mathematica and try to define a simple recursive formula which is illustrated as below:

A[1] := {{0, 0}, {0, 0}} // MatrixForm;
identity := {{1, 0}, {0, 1}};
A[n_] := KroneckerProduct[identity, A[n - 1]];
A[2] // MatrixForm

The result is expected to be a zero matrix with size 4$\times$4, i.e.

$$ \begin{pmatrix} 0&0&0&0\\ 0&0&0&0\\ 0&0&0&0\\ 0&0&0&0 \end{pmatrix} $$

if I let n=2. However, the returned result is

$$ \begin{pmatrix} 0&0&0&1\\ 0&0&1&0\\ 0&1&0&0\\ 1&0&0&0 \end{pmatrix} $$

I have checked many times but still don't know what's the error for my code. Could anyone give some hints?

Don't include display wrappers (e.g., MatrixForm) in variable definitions. Use either MatrixForm[A[1] = ... ] or (A[1] = ... )//MatrixForm to isolate the wrapper from the variable definition. — Bob Hanlon, Sep 28 '21 at 05:39

score 2 · Answer 1 · answered Sep 28 '21 at 05:40

Notice use of parentheses on line 1 and also on line 4. MatrixForm outputs cannot be used in computations.

(A[1] := {{0, 0}, {0, 0}}) // MatrixForm;
identity := {{1, 0}, {0, 1}};
A[n_] := KroneckerProduct[identity, A[n - 1]];
(A[2]) // MatrixForm

\begin{array}{cccc} 0 & 0 & 0 & 0 \\ 0 & 0 & 0 & 0 \\ 0 & 0 & 0 & 0 \\ 0 & 0 & 0 & 0 \\ \end{array}

1 Answers1