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

1,3

Links

Table of n, a(n) for n=1..44.

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

Example

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.

Prog

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

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.

Sequence in context: A216624 A183413 A183380 * A095370 A046053 A368303

Adjacent sequences: A260584 A260585 A260586 * A260588 A260589 A260590

Keyword

nonn,base,hard,more

Author

M. F. Hasler, Jul 29 2015

Extensions

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

Status

approved