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 A139257 #25 Oct 21 2024 04:36:17 %S A139257 6,14,62,254,16382,262142,1048574,4294967294,4611686018427387902, %T A139257 1237940039285380274899124222,324518553658426726783156020576254, %U A139257 340282366920938463463374607431768211454 %N A139257 Twice Mersenne primes A000668(n). %C A139257 Radicals of even perfect numbers. - _Charles R Greathouse IV_, Feb 01 2013 %H A139257 Amiram Eldar, <a href="/A139257/b139257.txt">Table of n, a(n) for n = 1..18</a> %H A139257 Florian Luca and Carl Pomerance, <a href="http://nyjm.albany.edu/j/2010/16-3.html">On the radical of a perfect number</a>, New York Journal of Math., 16 (2010), 23-30; <a href="http://www.math.dartmouth.edu/~carlp/LucaPomeranceNYJMstyle.pdf">alternative link</a>. %F A139257 a(n) = 2*A000668(n). %F A139257 a(n) = A000918(1 + A000043(n)) = A095121(A000043(n)). - _Omar E. Pol_, Jun 07 2012 %t A139257 2*(2^MersennePrimeExponent[Range[15]]-1) (* _Harvey P. Dale_, Jan 05 2020 *) %o A139257 (PARI) apply(p->2*(2^p-1),select(p->ispseudoprime(2^p-1),primes(40))) \\ _Charles R Greathouse IV_, Feb 01 2013 %Y A139257 Cf. A000043, A000668, A000918, A060590, A095121, A100484, A139256. %K A139257 nonn %O A139257 1,1 %A A139257 _Omar E. Pol_, Apr 23 2008 %E A139257 Corrected and extended by _Joerg Arndt_, Jun 07 2012.