After initially offering such marvellous calculators as the fx-201P or the FX-602P, Casio went steadily downhill; most of their later programmable models have extremely limited programmability, both in terms of capacity and in terms of functionality.

Not so the fx-4000P. This is a no-compromise, high-end, classic programmable. With a full set of scientific functions and a whopping 550-step program memory, this is definitely a serious engineering tool. They just don't make machines like this anymore!

I first came across the fx-4000P in its OEM guise as the Radio Shack EC-4020. At the time, I did not yet know about the fx-4000P model number, so the find represented a bit of a mystery.

In 2012, some 13 years after I received my first EC-4020 and 11 years after I came across the first fx-4000P, a kind calculator owner, Aron from NJ sent me his old fx-4000P, this time with an English manual. At last! (Well, I did have a German manual and the English manual for the EC-4020, but it's not quite like the real thing.) I must say, it is still a lovely, very capable machine; my only two complaints are the somewhat hard to read display and a less than ideally robust construction.

The program below, intended to demonstrate the fx-4000P's programming model, uses the Lanczos-approximation to accurately compute the logarithm of the Gamma function. A conditional expression is used to compute the correct result for negative arguments (the machine must be in radians mode; also note the difference between minus signs, '-', and subtraction signs, '−'):

Ans→X:
Abs X→Z:
ln (2.506628275+6.3E-10+(225.5255846+1.9E-8)÷Z−
(268.2959738+4.1E-8)÷(Z+1)+(80.90308069+3.5E-9)÷(Z+2)−
(5.007578639+7.1E-10)÷(Z+3)+(1.146848954E-2+3.5E-12)÷(Z+4))+
(Z−.5)ln (Z+4.65)−Z−4.65→G:
X<0⇒ln (-π÷X÷sin πX)−G→G:G