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 A125239 Smallest prime divisor of 10*T(n)+1 = 5*n*(n+1)+1, where T(n) = 1 + 2 + ... + n. 1
 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 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 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. LINKS Harvey P. Dale, Table of n, a(n) for n = 1..1000 N. Hobson, Triangular Numbers. EXAMPLE 10*T(9) + 1 = 5*9*10 + 1 = 451 = 11*41, so a(9) = 11. MATHEMATICA FactorInteger[#][[1, 1]]&/@(10*Accumulate[Range[60]]+1) (* Harvey P. Dale, Dec 12 2011 *) PROG (PARI) a(n) = if(n<1, 0, factor(5*n*(n+1)+1)[1, 1]) CROSSREFS Cf. A000217, A062786, A090562, A124989. Sequence in context: A040162 A113747 A202007 * A062786 A090562 A174244 Adjacent sequences: A125236 A125237 A125238 * A125240 A125241 A125242 KEYWORD easy,nonn AUTHOR Nick Hobson, Nov 25 2006 STATUS approved

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Last modified January 29 00:02 EST 2023. Contains 359905 sequences. (Running on oeis4.)