|
Search: id:A125239
|
|
|
| A125239 |
|
Smallest prime divisor of 10T(n)+1 = 5n(n+1)+1, where T(n) = 1+2+...+n. |
|
+0
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; listen)
|
|
|
OFFSET
|
1,1
|
|
|
COMMENT
|
All divisors of 10T(n)+1 are congruent to 1 or -1 modulo 10; that is, they end in the decimal digit 1 or 9.
|
|
LINKS
|
N. Hobson, Triangular Numbers.
|
|
EXAMPLE
|
10T(9)+1 = 5*9*10+1 = 451 = 11*41, so a(9) = 11.
|
|
PROGRAM
|
(PARI) a(n) = if(n<1, 0, factor(5*n*(n+1)+1)[1, 1])
|
|
CROSSREFS
|
Cf. A000217, A062786, A090562, A124989.
Sequence in context: A087394 A040162 A113747 this_sequence A062786 A090562 A136061
Adjacent sequences: A125236 A125237 A125238 this_sequence A125240 A125241 A125242
|
|
KEYWORD
|
easy,nonn
|
|
AUTHOR
|
Nick Hobson Nov 25 2006
|
|
|
Search completed in 0.002 seconds
|
