A260587 Number of distinct prime factors of A173426(n) = concatenation of (1, 2, ..., n, n-1, ..., 1).

0, 1, 2, 2, 2, 5, 2, 4, 3, 1, 2, 3, 3, 6, 6, 4, 4, 4, 6, 2, 5, 3, 4, 8, 2, 6, 8, 2, 4, 9, 4, 9, 6, 6, 6, 7, 3, 5, 7, 4, 6, 6, 3, 6

Offset: 1

Author

M. F. Hasler, Jul 29 2015

Keywords

• nonn
• base
• hard
• more

Examples

			a(2) = 1 since A173426(2) = 121 = 11*11 has only one distinct prime factor, 11.
a(21) = 5 since A173426(21) = 3^2 * 7 * 828703 * 94364768151913037621 * 250591098443370396365457961250972909.
a(25) = 2 since A173426(25) = A075023(n) * A075024(n) is a semiprime.
		

Links

• FactorDB, (121*10^(4*n-19) - 1002*10^(4*n-28) - 2*10^(2*n-9) + 879*10^10 + 121)/99^2.

Crossrefs

Cf. A001221.

See A260588 for the variant where prime factors are counted with multiplicity.

See also A075023 and A075024 for the smallest and largest prime factor of the terms.

Programs

PARI

a(n)=omega(A173426(n))

Extensions

a(38)-a(44) added using factordb.com by Jinyuan Wang, Mar 05 2020