A249224 Lpf (n(n+11)/2): least prime dividing n(n+11)/2.

2, 13, 3, 2, 2, 3, 3, 2, 2, 3, 11, 2, 2, 5, 3, 2, 2, 3, 3, 2, 2, 3, 17, 2, 2, 13, 3, 2, 2, 3, 3, 2, 2, 3, 5, 2, 2, 7, 3, 2, 2, 3, 3, 2, 2, 3, 29, 2, 2, 5, 3, 2, 2, 3, 3, 2, 2, 3, 5, 2, 2, 31, 3, 2, 2, 3, 3, 2, 2, 3, 41, 2, 2, 5, 3, 2, 2, 3, 3, 2, 2, 3, 47, 2, 2, 43, 3, 2, 2, 3, 3, 2, 2, 3, 5, 2, 2, 7, 3, 2

Offset: 1

Author

Zak Seidov, Oct 23 2014

Keywords

• nonn
• easy

Comments

a(n) = 2 for n = {4,5,8,9} (mod 12) and a(n) = 3 for n = {3,6,7,10} (mod 12).

Links

• Zak Seidov, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A056115, A020639.

Programs

Mathematica

FactorInteger[#(11+#)/2][[1,1]]& /@ Range[100]

PARI

a(n) = my(f=factor(n*(n+11)/2)); f[1,1]; \\ Michel Marcus, Oct 23 2014

Formula

a(n) = A020639(n(n+11)/2).