login
A264440
Row lengths of the irregular triangle A137510 (number of divisors d of n with 1 < d < n, or 0 if no such d exists).
2
1, 1, 1, 1, 1, 2, 1, 2, 1, 2, 1, 4, 1, 2, 2, 3, 1, 4, 1, 4, 2, 2, 1, 6, 1, 2, 2, 4, 1, 6, 1, 4, 2, 2, 2, 7, 1, 2, 2, 6, 1, 6, 1, 4, 4, 2, 1, 8, 1, 4, 2, 4, 1, 6, 2, 6, 2, 2, 1, 10, 1, 2, 4, 5, 2, 6, 1, 4, 2, 6, 1, 10, 1, 2, 4, 4, 2, 6, 1, 8, 3, 2, 1, 10, 2, 2
OFFSET
1,6
COMMENTS
See A032741 for the number of divisors d of n with 1 <= d < n, n >= 1.
See A070824 for the number of the divisors d of n with 1 < d < n, n >= 1.
LINKS
FORMULA
a(1) = 1; a(n) = 1 if n is prime, otherwise a(n) = A070824(n).
a(1) = 1; a(n) = 1 if n is prime, otherwise a(n) = A032741(n) - 1.
a(n) = max(1, A000005(n)-2). - Robert Israel, Jan 20 2016
MAPLE
seq(max(1, numtheory:-tau(n)-2), n=1..100); # Robert Israel, Jan 20 2016
MATHEMATICA
Array[DivisorSigma[0, #] - 2 &, {80}] /. n_ /; n < 2 -> 1 (* Michael De Vlieger, Jan 16 2016 *)
PROG
(PARI) A264440(n) = max(1, numdiv(n)-2); \\ After Robert Israel's formula. - Antti Karttunen, May 25 2017
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Wolfdieter Lang, Jan 16 2016
STATUS
approved