Also, the modifier for Representation uses an additional RoundDown function that effectively increases the Representation discount from 33% to 40%. See
http://forums.civfanatics.com/showpost.php?p=13126363&postcount=11 and subsequent posts.
The rounding functions are actually just the result of integer division in C++ and are in slightly different places. In Excel language, the culture cost function would be:
FLOOR((25 + FLOOR((3 * [non-free policies unlocked])^2.01, 1))) * (1 + FLOOR(FLOOR(10*(100-33)/100, 1) * ([num cities] - 1) / 100, 1)))*[policy cost mod]*[game speed mod]*[handicap mod], 5).
The equation is therefore the following, though the constant round-downs do introduce some inaccuracies:
Replacing sums with finite integrals is not accurate at the small amount of sum terms I would be replacing, I've opted to run my calculations using the actual sum with the help of some scripting.
Actual stats are as follows, once again assuming n(x) is constant: k = 2 is 17 cities, k = 4 is 27 cities, and k = 5 is 44 cities. If we assume 2 extra policies before Rationalism and Liberty are completed (almost always the case by Atomic), k = 2 is 12 cities, k = 4 is 15 cities, and k = 5 is 18 cities. If we assume 4 extra policies before those two policy trees are completed (usually the case for Liberty by Information Era), k = 2 is 9 cities, k = 4 is 10 cities, and k = 5 is 12 cities, with these values decreasing by about 1 for each extra policy afterwards (5 extra policies in addition to full Liberty and full Rationalism is 8, 9, and 10, and 6 extra is 7, 8, 9). Remember, this is also all assuming you're always siting on this many cities from the turn that Representation is unlocked: gaining or losing cities at various points in the game can push your free policy's worth of culture saved back or forward quite a bit. Basically, you'll always need to wait for Information Era before getting your free policy's worth of saved up culture.