The HP-15C was a high end scientific programmable. To the HP-34C's numerical integration and root solver the HP-15C added complex numbers and matrix calculations.
Complex Numbers
A first on the HP-15C was built-in support for complex numbers. Programs for manipulating complex numbers existed as far back as the HP-65. To use these programs, the user entered two numbers into two stack positions and invoked the appropriate subroutine. This was cumbersome because it turned a four level stack into a two level stack and required a subroutine (and label) for any complex function the user might require.
By contrast, when the HP-15C entered Complex mode, it consumed 5 storage registers, using four to add imaginary components to each level of the stack and one to add the imaginary component to Last X.
