A104524 - OEIS

A104524

Number of distinct prime divisors of 55...557 (with n 5s).

2, 1, 1, 2, 1, 3, 3, 3, 1, 3, 2, 5, 3, 1, 4, 3, 3, 3, 4, 2, 1, 2, 7, 4, 4, 4, 4, 5, 4, 4, 3, 2, 3, 5, 3, 4, 3, 7, 2, 4, 2, 4, 4, 8, 2, 4, 3, 6, 3, 2, 4, 7, 7, 7, 5, 5, 6, 7, 2, 4, 6, 3, 5, 5, 2, 5, 2, 4, 5, 5, 3, 9, 10, 6, 5, 6, 4, 4, 4, 5, 4, 5, 3, 6, 6, 4, 1

(list; graph; refs; listen; history; text; internal format)

OFFSET

1,1

LINKS

Amiram Eldar, Table of n, a(n) for n = 1..192

FORMULA

a(n) = A001221(A178769(n+1)). - R. J. Mathar, Aug 24 2011

EXAMPLE

The number of distinct prime divisors of 57 is 2.

The number of distinct prime divisors of 557 is 1 (prime).

The number of distinct prime divisors of 5557 is 1 (prime).

MAPLE

A104524 := proc(n) local x ; x := [7, seq(5, k=1..n)] ; add(op(i, x)*10^(i-1), i=1..nops(x)) ; numtheory[factorset](%) ; nops(%) ; end proc: # R. J. Mathar, Aug 24 2011

MATHEMATICA

Table[PrimeNu[(50*10^n + 13)/9], {n, 1, 50}](* G. C. Greubel, May 07 2017 *)

Table[PrimeNu[FromDigits[PadLeft[{7}, n, 5]]], {n, 2, 90}] (* Harvey P. Dale, Dec 03 2021 *)

PROG

(PARI) a(n) = omega((5*10^(n+1)+13)/9); \\ Michel Marcus, May 08 2017

CROSSREFS

Cf. A104484, A104483, A104518.

Cf. A001221, A178769.

Sequence in context: A105248 A289495 A076302 * A233322 A233324 A268679

Adjacent sequences: A104521 A104522 A104523 * A104525 A104526 A104527

KEYWORD

nonn,base

AUTHOR

Parthasarathy Nambi, Apr 20 2005

EXTENSIONS

More terms from Michel Marcus, May 08 2017

More terms from Amiram Eldar, Jan 25 2020

STATUS

approved