A260587 - OEIS

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

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OFFSET

1,3

LINKS

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

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.

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