The HP-48SX was the first of HP's current line of high-end graphic calculators. It was essentially an upgraded version of the HP-28S, with many new functions, a real-time clock and calendar, two extension ports that could accept ROM or RAM modules, a bidirectional infrared interface and an RS-232 port used to exchange data with other computers.

On the exterior, the HP-48SX looks more like traditional HP calculators; gone is the controversial clamshell design. The keyboard is very crowded, with three shift keys (gold, blue, and alpha); even so, most calculator functions can be accessed through soft keys only. The calculator itself feels sturdy, with easily accessible compartments for battery and extension cards. Despite the crowded keyboard, it's obviously a much better design than the HP-28S.

This calculator is not really a collector's item, not yet; it is, in fact, the calculator I've been using on an everyday basis for the last 6-7 years. Before I bought a portable computer, I actually used this machine as a personal organizer of sorts, with programs of my own creation, for taking financial data or addresses with me on the road. I also experimented with assembly language programming using tools released by HP, as well as some excellent third-party tools released into the public domain by other enthusiastic HP-48SX users. I remain very fond of this device, despite the fact that I still have trouble remembering the four-key sequence needed to recall the value of pi (it's gold-shift, pi, blue-shift, ->NUM, in case you're wondering.)

As a high-end scientific/engineering calculator, the HP-48SX has a built-in implementation of the Gamma function. More accurately, the calculator can calculate an extended factorial for any real number; the Gamma function of any real x can be obtained as the factorial of x-1.

Built-in Gamma function notwithstanding, my implementation can still be useful. The built-in version can only calculate the Gamma function for reals; the implementation shown here extends the definition of the function to the entire complex plane.

«
  -> x
  «
    x 1
    WHILE OVER RE 0 < REPEAT
      OVER * SWAP 1 + SWAP
    END
    SWAP DUP 1 + 76.1800917295 SWAP /
    OVER 2 + 86.5053203294 SWAP / -
    OVER 3 + 24.0140982408 SWAP / +
    OVER 4 + 1.23173957245 SWAP / -
    OVER 5 + 1.20865097387E-3 SWAP / +
    OVER 6 + 5.39523938495E-6 SWAP / -
    1.00000000019 + OVER  / π ->NUM 2 * √
    * LN SWAP DUP DUP 5.5 + LN SWAP .5 + *
    SWAP 5.5 + - + EXP SWAP /
  »
»