The OEIS mourns the passing of Jim Simons and is grateful to the Simons Foundation for its support of research in many branches of science, including the OEIS.
The OEIS is supported by the many generous donors to the OEIS Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 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 Nick 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

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recents
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified June 15 14:44 EDT 2024. Contains 373407 sequences. (Running on oeis4.)