I am looking for questions of two types.

1. How many occurrences of these(types) are there? Usually the total count does not fit into a 32 bit integer and the time complexity is much smaller than the count. Are these called combinatorics (I guess)
2. I am also interested in series. Sometimes we have series like "1561117..." There is a series inside 1,5,6,11,17 and the job is find the ith character in the string. Sometimes its other way round. Mapping from one type of index to other in a series. Is there a question set about series?