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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 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 LINKS Michael De Vlieger, Table of n, a(n) for n = 1..10000 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. A000040, A001358, A020639, A076396, A085155. 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 Adjacent sequences: A122440 A122441 A122442 * A122444 A122445 A122446 KEYWORD easy,nonn AUTHOR Jonathan Vos Post, Sep 06 2006 EXTENSIONS More terms from Michel Marcus, Mar 04 2017 STATUS approved

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Last modified September 13 03:33 EDT 2024. Contains 375857 sequences. (Running on oeis4.)