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a(n) = p*(p - 1), where p is A000043(n).
5

%I #21 Oct 18 2024 11:42:03

%S 2,6,20,42,156,272,342,930,3660,7832,11342,16002,270920,367842,

%T 1634562,4851006,5200680,10345872,18083756,19558506,93867032,98813540,

%U 125720156,397464032,470911700,538634472,1979938512,7437768806,12210802506

%N a(n) = p*(p - 1), where p is A000043(n).

%H Amiram Eldar, <a href="/A139115/b139115.txt">Table of n, a(n) for n = 1..48</a> (terms 1..47 from Harvey P. Dale)

%F a(n) = A000043(n)*(A000043(n) - 1).

%t #(#-1)&/@MersennePrimeExponent[Range[30]] (* Requires Mathematica version 10 or later *) (* _Harvey P. Dale_, Jan 15 2020 *)

%Y Cf. A000043, A036689, A139306, A139307, A139116, A139223, A139224, A139225, A139226.

%K nonn

%O 1,1

%A _Omar E. Pol_, May 10 2008

%E More terms from _Vincenzo Librandi_, May 11 2010