Nth number in a infinite sequence of numbers
The idea here is as follows:


The first 9 digits are 123456789.
The next 90 pairs of digits are (10)(11)...(99).
The following 900 triplets of digits are (100)(101)...(999).
And so on.


Given an index $n$, you calculate which group you are (singles, pairs, triplets, ...), then which tuple you belong to and which digit inside the tuple. For example, if $n=28$ then this is digit no. 19 in the second group, which is the first digit of the 10th tuple 19, that is, 1.