cp's OEIS Frontend

This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.

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A125239 Smallest prime divisor of 10*T(n)+1 = 5*n*(n+1)+1, where T(n) = 1 + 2 + ... + n.

Original entry on oeis.org

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

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Author

Nick Hobson, Nov 25 2006

Keywords

Comments

All divisors of 10*T(n)+1 are congruent to 1 or -1 modulo 10; that is, they end in the decimal digit 1 or 9.

Examples

			10*T(9) + 1 = 5*9*10 + 1 = 451 = 11*41, so a(9) = 11.
		

Crossrefs

Programs

  • Mathematica
    FactorInteger[#][[1,1]]&/@(10*Accumulate[Range[60]]+1) (* Harvey P. Dale, Dec 12 2011 *)
  • PARI
    a(n) = if(n<1, 0, factor(5*n*(n+1)+1)[1,1])
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