OFFSET
1,1
COMMENTS
a(n) <= 2p, where p = A002586(4n) is the least prime factor of (1 + 16^n). (See the Comment in A066135.) - Jonathan Sondow, Nov 23 2012
LINKS
Michael De Vlieger, Table of n, a(n) for n = 1..1000
FORMULA
a(n) = Min{x : sigma_4n(x) mod x = 0, x > 1}
EXAMPLE
First 3 terms correspond to entries of other sequences as follows: a(1)=A046763(2), a(2)=A055712(2), a(3)=A055716(2).
From Michael De Vlieger, Jul 17 2017: (Start)
First position of values, with observations pertaining to values for 1 <= n <= 3000:
Value Position Observations:
--------------------------------
34 1 All odd.
84 2 In A047235.
194 6 In A017593.
228 12
386 36
1282 72
1538 144
3084 288
147468 576
1956 864
1046532 1152
24578 2304
3252 2880
(End)
MATHEMATICA
Table[m = 2; While[Mod[DivisorSigma[4 n, m], m] > 0, m++]; m, {n, 66}] (* Michael De Vlieger, Jul 17 2017 *)
PROG
(PARI) a(n) = {n *= 4; my(m = 2); while (sigma(m, n) % m, m++); m; } \\ Michel Marcus, Oct 02 2016
CROSSREFS
KEYWORD
nonn
AUTHOR
Labos Elemer, Dec 11 2001
STATUS
approved