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Product of smallest and greatest prime factors of n.
18

%I #20 Jun 17 2018 01:57:07

%S 1,4,9,4,25,6,49,4,9,10,121,6,169,14,15,4,289,6,361,10,21,22,529,6,25,

%T 26,9,14,841,10,961,4,33,34,35,6,1369,38,39,10,1681,14,1849,22,15,46,

%U 2209,6,49,10,51,26,2809,6,55,14,57,58,3481,10,3721,62,21,4,65,22,4489,34

%N Product of smallest and greatest prime factors of n.

%C For all n > 1, a(n) is semiprime; n is semiprime iff a(n) = n; n is prime iff a(n) = n^2;

%C For all primes p and k > 0, a(p^k) = p^2.

%H R. Zumkeller, <a href="/A066048/b066048.txt">Table of n, a(n) for n = 1..10000</a>

%F a(n) = A020639(n) * A006530(n).

%t f[n_] := Transpose[ FactorInteger[n]][[1]]; Table[ First[f[n]]*Last[f[n]], {n, 2, 100} ]

%t p[n_]:=Module[{fctrs=Transpose[FactorInteger[n]][[1]]},First[fctrs] Last[fctrs]]; Array[p,70] (* _Harvey P. Dale_, Jun 11 2011 *)

%o (PARI) { for (n = 1, 10000, if (n==1, a=1, f=factor(n)~; a=f[1, 1] * f[1, length(f)]); write("b066048.txt", n, " ", a) ) } \\ _Harry J. Smith_, Nov 08 2009

%Y Cf. A000040, A001358, A001248, A000961.

%Y Cf. A130064, A130065.

%K nonn,nice,easy

%O 1,2

%A _Reinhard Zumkeller_

%E More terms from _Robert G. Wilson v_, Jan 03 2002