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A124874 Number of distinct primes dividing A007408(n). 1
0, 1, 1, 3, 2, 3, 3, 2, 3, 4, 3, 2, 3, 2, 1, 3, 2, 4, 1, 3, 2, 5, 2, 4, 2, 1, 3, 4, 3, 5, 1, 2, 3, 2, 3, 8, 4, 4, 5, 4, 8, 6, 7, 2, 4, 6, 3, 2, 4, 3, 2, 4, 3, 4, 2, 8, 3, 3, 4, 4, 3, 3, 7, 2, 5, 9, 4, 2, 6, 7, 3, 5, 6, 4, 3, 6, 4, 6, 4, 4, 3, 3, 8, 5, 7, 4, 5 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,4

LINKS

Amiram Eldar, Table of n, a(n) for n = 1..106

FORMULA

a(n) = A001221(A007408(n)).

EXAMPLE

a(1) = 0 since no prime divides A007408(1) = 1,

a(2) = 1 since 3 is the only prime dividing A007408(2) = 9,

a(3) = 1 since A007408(3) = 251 is prime,

a(4) = 3 since A007408(4) = 5 * 11 * 37,

a(6) = 3 since 7, 11, 53 are the only primes dividing A007408(6) = 7^2 * 11 * 53.

MAPLE

seq( (nops@op)(2, (ifactors@A007408)(n)), n=1..25);

MATHEMATICA

Table[PrimeNu[Numerator[Sum[1/k^3, {k, 1, n}]]], {n, 1, 50}] (* Amiram Eldar, Feb 09 2020 *)

CROSSREFS

Cf. A001221, A007408, A124876, A124877.

Sequence in context: A054263 A308656 A178775 * A230258 A016459 A242309

Adjacent sequences:  A124871 A124872 A124873 * A124875 A124876 A124877

KEYWORD

nonn

AUTHOR

M. F. Hasler, Nov 11 2006

EXTENSIONS

More terms from Amiram Eldar, Feb 09 2020

STATUS

approved

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Last modified March 28 20:01 EDT 2020. Contains 333103 sequences. (Running on oeis4.)