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 A308480 #15 Jul 18 2019 07:45:50 %S A308480 3,7,2,31,2,127,2,3,2,2047,2,8191,2,3,2,131071,2,524287,2,3,2,8388607, %T A308480 2,5,2,3,2,536870911,2,2147483647,2,3,2,5,2,137438953471,2,3,2, %U A308480 2199023255551,2,8796093022207,2,3,2,140737488355327,2,7,2,3,2,9007199254740991 %N A308480 a(n) = A000225(n) if n is prime, a(n) = A020639(n) otherwise. %C A308480 What is the asymptotic behavior of the sequences defined by the recursive map x -> a(x)? Do these sequences increase without bound or do they enter a repeating cycle? %C A308480 For example, the trajectory of 11 under the above map starts 11, 2047, 23, 8388607, 47, 140737488355327, 2351, s, 4703, t, ..., where s is a 708-digit number and t is a 1416-digit number. t has no prime factor less than 2^64 (cf. GIMPS link). %H A308480 GIMPS, <a href="https://www.mersenne.org/report_exponent/?exp_lo=4703&exp_hi=&full=1">PrimeNet Exponent Status for M4703</a> %o A308480 (PARI) a(n) = if(ispseudoprime(n), 2^n-1, factor(n)[1, 1]) %Y A308480 Cf. A000225, A006370, A020639. %K A308480 nonn %O A308480 2,1 %A A308480 _Felix Fröhlich_, May 30 2019