Katapayadi sankhya is a simplification of Aryabhata's Sanskrit numerals, due probably to Haridatta from Kerala. In Malayalam it is also known as 'Paralperu'
For eg: ചണ്ഡാംശുചന്ദ്രാധമകുംഭിപാല
represents 31415926536 which is π*1000000000000000
More examples in Malayalam are given in this page
The Katapayadi sankhya is a way of determining the number of a Melakarta raga from the first two
syllables of the name of the Raga.
The algorithm used in this is explained in detail here: An Algorithm to derive the Swarasthanas