A104517 - OEIS

A104517

Number of distinct prime divisors of 55...1 (with n 5s).

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

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

OFFSET

1,1

COMMENTS

Number of distinct prime factors of (10^(n + 1) - 1)*5/9 - 4. - Stefan Steinerberger, Mar 06 2006

LINKS

Amiram Eldar, Table of n, a(n) for n = 1..200

FORMULA

a(n) = A001221(A173804(n+1)). - Amiram Eldar, Jan 24 2020

EXAMPLE

The number of distinct prime divisors of 51 is 2 which is the first term in the sequence.

The number of distinct prime divisors of 551 is 2 which is the second term in the sequence.

The number of distinct prime divisors of 5551 is 3 which is the third term in the sequence.

MAPLE

f:= n -> nops(numtheory:-factorset( (10^(n + 1) - 1)*5/9 - 4)):

map(f, [$1..92]); # Robert Israel, Mar 08 2018

MATHEMATICA

Table[Length[FactorInteger[(10^(n + 1) - 1)*5/9 - 4]], {n, 1, 50}] (* Stefan Steinerberger, Mar 06 2006 *)

PROG

(Magma) [#PrimeDivisors((10^(n+1)-1)*5 div 9-4): n in [1..80]]; // Vincenzo Librandi, Mar 09 2018

(PARI) a(n) = omega((10^(n + 1) - 1)*5/9 - 4); \\ Michel Marcus, Mar 09 2018

CROSSREFS

Cf. A001221, A056684 (a(n)=1), A104484, A173804.

Sequence in context: A214861 A037914 A073813 * A098397 A278744 A346213

Adjacent sequences: A104514 A104515 A104516 * A104518 A104519 A104520

KEYWORD

nonn,base

AUTHOR

Parthasarathy Nambi, Apr 19 2005

EXTENSIONS

More terms from Stefan Steinerberger, Mar 06 2006

a(51)-a(92), and offset corrected, by Robert Israel, Mar 08 2018

STATUS

approved