A125239 Smallest prime divisor of 10*T(n)+1 = 5*n*(n+1)+1, where T(n) = 1 + 2 + ... + n.
11, 31, 61, 101, 151, 211, 281, 19, 11, 19, 661, 11, 911, 1051, 1201, 1361, 1531, 29, 1901, 11, 2311, 2531, 11, 3001, 3251, 3511, 19, 31, 19, 4651, 11, 5281, 31, 11, 6301, 6661, 79, 7411, 29, 59, 79, 11, 9461, 9901, 11, 19, 29, 19, 12251, 41, 89, 13781, 11
Offset: 1
Examples
10*T(9) + 1 = 5*9*10 + 1 = 451 = 11*41, so a(9) = 11.
Links
- Harvey P. Dale, Table of n, a(n) for n = 1..1000
- Nick Hobson, Triangular Numbers.
Programs
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Mathematica
FactorInteger[#][[1,1]]&/@(10*Accumulate[Range[60]]+1) (* Harvey P. Dale, Dec 12 2011 *)
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PARI
a(n) = if(n<1, 0, factor(5*n*(n+1)+1)[1,1])
Comments