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A122443
Least prime factor of powers of semiprimes.
2
1, 2, 2, 3, 2, 2, 3, 2, 3, 2, 5, 2, 3, 2, 5, 2, 2, 3, 2, 7, 3, 5, 3, 2, 2, 2, 5, 3, 2, 7, 3, 2, 5, 2, 3, 7, 3, 2, 5, 2, 2, 3, 5, 2, 7, 11, 2, 3, 3, 7, 2, 3, 2, 11, 5, 2, 5, 2, 3, 7, 2, 13, 3, 2, 3, 5, 11, 2, 2, 3, 2, 7, 5, 2, 11, 3, 2, 5, 2, 7, 2, 3, 13, 3, 2, 5, 3, 13
OFFSET
1,2
LINKS
FORMULA
a(n) = A020639(A085155(n)) = least prime factor of A085155 powers of semiprimes.
MATHEMATICA
FactorInteger[#][[1, 1]] & /@ Select[Range@ 250, Function[n, Or[n == 1, And[Length@ # == 1, EvenQ@ First@ #], And[Length@ # == 2, SameQ @@ #]] &[FactorInteger[n][[All, -1]]]]] (* Michael De Vlieger, Mar 04 2017 *)
PROG
(PARI) is(n)=my(f=factor(n)[, 2]); #f==0 || (#f==2 && f[1]==f[2]) || (#f==1 && f[1]%2==0);
spf(n) = if (n==1, 1, factor(n)[1, 1]);
lista(nn) = {for (n=1, nn, if (is(n), print1(spf(n), ", ")); ); } \\ Michel Marcus, Mar 04 2017
CROSSREFS
Cf. A122444 (greatest prime factor of powers of semiprimes).
Cf. A076396 (smallest prime factor of n-th perfect power).
Sequence in context: A046028 A330406 A125954 * A262945 A309674 A270516
KEYWORD
easy,nonn
AUTHOR
Jonathan Vos Post, Sep 06 2006
EXTENSIONS
More terms from Michel Marcus, Mar 04 2017
STATUS
approved