HP 15c - Manual (Page 173)

Section 12: Calculating with Matrices 173
A problem using this procedure is given in the HP-15C Advanced Functions
Handbook under Solving a Large System of Complex Equations.
Miscellaneous Operations Involving Matrices
Using a Matrix Element With Register Operations
If a letter key specifying a matrix is pressed after any of the following
function keys, the operation is performed using the matrix element specified
by the row and column numbers in R
0
and R
1
, just as though it were a data
storage register.
O * l *
O{+, -, *, ÷} l{+
, - , * ,
÷}
e I
X
Using Matrix Descriptors in the Index Register
In certain applications, you may want to perform a programmed sequence
of matrix operations using any of the matrices A through E. In this
situation, the matrix operations can refer to whatever matrix descriptor is
stored in the index register (R
I
).
If the Index register contains a matrix descriptor:

Pressing % after any of the functions listed above performs the
operations using the element specified by R
0
and R
1
and the matrix
specified in R
I
.

Pressing % after O| or l| performs the operation
using the element specified by the row and column numbers in the
Y- and X-registers and the matrix specified in R
I.
* Also, in User mode the row and column numbers in R
0
and R
1
are incremented according to the
dimensions of the specified matrix.