cp's OEIS Frontend

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

%I A260587 #21 Mar 05 2020 03:38:30
%S A260587 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,
%T A260587 6,7,3,5,7,4,6,6,3,6
%N A260587 Number of distinct prime factors of A173426(n) = concatenation of (1, 2, ..., n, n-1, ..., 1).
%H A260587 FactorDB, <a href="http://factordb.com/index.php?query=%28121*10%5E%284*n-19%29+-+1002*10%5E%284*n-28%29+-+2*10%5E%282*n-9%29+%2B+879*10%5E10+%2B+121%29%2F99%5E2&amp;perpage=60">(121*10^(4*n-19) - 1002*10^(4*n-28) - 2*10^(2*n-9) + 879*10^10 + 121)/99^2</a>.
%e A260587 a(2) = 1 since A173426(2) = 121 = 11*11 has only one distinct prime factor, 11.
%e A260587 a(21) = 5 since A173426(21) = 3^2 * 7 * 828703 * 94364768151913037621 * 250591098443370396365457961250972909.
%e A260587 a(25) = 2 since A173426(25) = A075023(n) * A075024(n) is a semiprime.
%o A260587 (PARI) a(n)=omega(A173426(n))
%Y A260587 Cf. A001221.
%Y A260587 See A260588 for the variant where prime factors are counted with multiplicity.
%Y A260587 See also A075023 and A075024 for the smallest and largest prime factor of the terms.
%K A260587 nonn,base,hard,more
%O A260587 1,3
%A A260587 _M. F. Hasler_, Jul 29 2015
%E A260587 a(38)-a(44) added using factordb.com by _Jinyuan Wang_, Mar 05 2020