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.
login
The OEIS is supported by the many generous donors to the OEIS Foundation.

 

Logo
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
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
Sequence in context: A040162 A113747 A202007 * A062786 A090562 A174244
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.

License Agreements, Terms of Use, Privacy Policy. .

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