The HP-28C was a short-lived version of Hewlett Packard's more popular HP-28S calculator. Unlike the HP-28S with its 32 kB of RAM, the HP-28C had only a miserable 2 kB. Otherwise, the two calculators were identical.

According to calculator folklore,  HP quickly realized that only 2 kB of memory is simply not enough to take advantage of the amazing capabilities this machine has to offer. Which is why, shortly after its introduction, the 28C was replaced with the 28S and disappeared form store shelves. Which may explain why this calculator is a bit difficult to find nowadays.

Well, on a calculator with too little memory, it only makes sense to save some, even if it's at the expense of processing time. So rather than presenting yet another copy of my usual Gamma function programming example, here's a program that calculates the incomplete Gamma function. This program can also be used to approximate the regular Gamma function by specifying a high enough integration limit; for instance, 5 ENTER 50 IGAMMA (assuming the program is saved under the name IGAMMA) yields 24, the exact value of the Gamma function of 5.

<< DUP2 SWAP ^ ROT SWAP OVER / DUP
   DO
     SWAP ROT 1 + SWAP OVER / 4 PICK *
     ROT DUP 3 PICK + SWAP OVER -
   UNTIL
     0 ==
   END
   ROT ROT DROP2 SWAP EXP /
>>