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%I #19 Mar 16 2022 03:19:48
%S 4,16,256,4096,16777216,4294967296,68719476736,1152921504606846976,
%T 1329227995784915872903807060280344576,
%U 95780971304118053647396689196894323976171195136475136,6582018229284824168619876730229402019930943462534319453394436096
%N a(n) = 2^(2p-1)/2, where p is A000043(n).
%C Ultraperfect numbers (A139306), divided by 2.
%C Also, a(n) is the largest proper divisor of the n-th ultraperfect number.
%C The cototient (A051953) of the even perfect numbers (A000396). - _Amiram Eldar_, Mar 06 2022
%C These cototients are squares = (2^(p-1))^2. - _Bernard Schott_, Mar 14 2022
%F a(n) = A139306(n)/2.
%F a(n) = A051953(A000396(n)), if there are no odd perfect numbers. - _Amiram Eldar_, Mar 06 2022
%F a(n) = A061652(n)^2. - _Bernard Schott_, Mar 14 2022
%t a[n_] := 4^(MersennePrimeExponent[n] - 1); Array[a, 12] (* _Amiram Eldar_, Mar 06 2022 *)
%Y Cf. A000043, A000396, A019279, A051953, A061652, A139288, A139306, A152922, A152923.
%K nonn
%O 1,1
%A _Omar E. Pol_, Dec 15 2008
%E More terms from _Amiram Eldar_, Mar 06 2022
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