negative number raised to even power

[Marcus von Cube]

(04-26-2015 12:47 PM)rprosperi Wrote:  HP-20S returns 4
HP-17BII+ returns 4 (Algebraic mode)
HP-10B returns 4

...and so I assume all HP algebraic machines (Pioneers at least) will behave the same way.

It depends on whether ^2 is a postfix function key that is executed directly on some displayed register value or if its entered on an algebraic command line. All the machines mentioned in your list work in the first way. But You can enter X=-2^2 in equation mode on the 17B and its successors and you should get -4 if you solve of X then.