Let N = 123456789123456789123456789..., with the digits from 1 to 9 repeated indefinitely. Then a(n) is the n-digit number formed from the digits starting at the {n(n-1)/2 +1}-th position of N, read backwards if n is even.

A066580

Let N = 123456789123456789123456789..., with the digits from 1 to 9 repeated indefinitely. Then a(n) is the n-digit number formed from the digits starting at the {n(n-1)/2 +1}-th position of N, read backwards if n is even.