Question

Sequence of natural numbers ending in 7 and 9

Answer 1

Step-by-step explanation:

This is the set of all natural numbers n such that n mod 10^m < 10^(m-1) for all natural numbers m.

Hence,

If T(k) were to denote the kth term of the series,

n = T(n-floor(n/10)-9*floor(n/100)-81*floor(n/1000)-....-9^(m-1)*floor(n/10^m)-...)

Hence, if we were to find the function which maps k to T(k), we have T(k) as the inverse of:

k-floor(k/10)-9*floor(k/100)-81*floor(k/1000)-....-9^(m-1)*floor(k/10^m)-...