This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A152921 #19 Mar 16 2022 03:19:48 %S A152921 4,16,256,4096,16777216,4294967296,68719476736,1152921504606846976, %T A152921 1329227995784915872903807060280344576, %U A152921 95780971304118053647396689196894323976171195136475136,6582018229284824168619876730229402019930943462534319453394436096 %N A152921 a(n) = 2^(2p-1)/2, where p is A000043(n). %C A152921 Ultraperfect numbers (A139306), divided by 2. %C A152921 Also, a(n) is the largest proper divisor of the n-th ultraperfect number. %C A152921 The cototient (A051953) of the even perfect numbers (A000396). - _Amiram Eldar_, Mar 06 2022 %C A152921 These cototients are squares = (2^(p-1))^2. - _Bernard Schott_, Mar 14 2022 %F A152921 a(n) = A139306(n)/2. %F A152921 a(n) = A051953(A000396(n)), if there are no odd perfect numbers. - _Amiram Eldar_, Mar 06 2022 %F A152921 a(n) = A061652(n)^2. - _Bernard Schott_, Mar 14 2022 %t A152921 a[n_] := 4^(MersennePrimeExponent[n] - 1); Array[a, 12] (* _Amiram Eldar_, Mar 06 2022 *) %Y A152921 Cf. A000043, A000396, A019279, A051953, A061652, A139288, A139306, A152922, A152923. %K A152921 nonn %O A152921 1,1 %A A152921 _Omar E. Pol_, Dec 15 2008 %E A152921 More terms from _Amiram Eldar_, Mar 06 2022