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

Offset: 1

Author

Parthasarathy Nambi, Apr 19 2005

Keywords

• nonn
• base

Comments

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

Examples

			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.
		

Links

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

Crossrefs

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

Programs

Magma

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

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 *)

PARI

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

Formula

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

Extensions

More terms from Stefan Steinerberger, Mar 06 2006

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