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