The Devil's Calculator

Here is an excerpt from The Devil's Calculator's description on Steam:

An evil calculator has its normal arithmetic operators replaced with sadistically obscure functions. Using logic and a bit of algebra, you must work backwards to interpolate and use these corrupted operators to compute the 'number of the beast', 666, in order to purge the calculator of its demons and continue on. Non-profit and open-source, The Devil's Calculator is a highly original indie game that's as addicting as it is educational.

Each level has you decipher the function of a mysterious operator and use it to calculate the number of the beast to progress. The game features advanced graphing mechanics and access to various resources to help you interpolate and solve the innovative and creative function level design.