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%I #15 Jan 29 2022 13:09:33
%S 1,1,1,1,1,2,1,1,1,2,1,2,1,2,5,1,1,1,1,2,1,2,1,2,1,2,1,2,1,2,1,1,1,2,
%T 5,3,1,2,1,2,1,2,1,2,1,2,1,2,1,5,1,2,1,2,1,2,1,2,1,2,1,2,7,1,5,2,1,2,
%U 1,2,1,1,1,2,3,2,1,2,1,2,1,2,1,2,5,2,1,2,1,2,1,2,1,2,1,2,1,1,1,1,1,2,1,2,5
%N Smallest common prime-divisor of n and Sigma_2(n) = A001157(n); a(n) = 1 if no common prime-divisor exists.
%H Antti Karttunen, <a href="/A082069/b082069.txt">Table of n, a(n) for n = 1..16384</a>
%F a(n) = A020639(A179930(n)). - _Antti Karttunen_, Nov 03 2017
%t ffi[x_] := Flatten[FactorInteger[x]]; lf[x_] := Length[FactorInteger[x]]; ba[x_] := Table[Part[ffi[x], 2*w-1], {w, 1, lf[x]}]; f1[x_] := n; f2[x_] := DivisorSigma[2, x]; Table[Min[Intersection[ba[f1[w]], ba[f2[w]]]], {w, 1, 128}]
%t (* Second program: *)
%t Array[If[CoprimeQ[#1, #2], 1, Min@ Apply[Intersection, Map[FactorInteger[#][[All, 1]] &, {#1, #2}]]] & @@ {#, DivisorSigma[2, #]} &, 105] (* _Michael De Vlieger_, Nov 03 2017 *)
%o (PARI)
%o A020639(n) = if(1==n,n,vecmin(factor(n)[, 1]));
%o A082069(n) = A020639(gcd(sigma(n,2), n)); \\ _Antti Karttunen_, Nov 03 2017
%Y Cf. A001157, A020639, A179930.
%Y Cf. also A082063, A082067, A082068, A082070, A082071, A082072.
%K nonn
%O 1,6
%A _Labos Elemer_, Apr 07 2003
%E Changed "was found" to "exists" in definition. - _N. J. A. Sloane_, Jan 29 2022
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