%I
%S 1,2,2,3,3,4,4,2,3,5,5,6,6,4,3,7,7,8,8,4,5,9,9,3,6,6,4,10,10,11,11,5,
%T 7,7,4,12,12,8,6,13,13,14,14,5,9,15,15,4,4,7,7,16,16,5,5,8,10,17,17,
%U 18,18,11,4,6,6,19,19,9,9,20,20,21,21,12,8,8,6
%N Index of the largest prime which divides n(n+1).
%C Yields the row of A145605 in which n appears, and also the first row of A138180 in which n appears.
%F a(n) = pi(A074399(n)), where pi = A000720.
%o (PARI) a(n)=primepi(vecmax(factor(n*(n+1))[,1]))
%Y Cf. A002071, A145604, A138180, A145605, A002072, A074399.
%K nonn
%O 1,2
%A _M. F. Hasler_, Jan 16 2015
