A332216 - OEIS

%I #33 Feb 13 2020 02:52:27

%S 2,3,5,7,11,19,23,31,47,55,79,87,127,191,383,1279,5119,6143,8191,

%T 20479,81919,131071,524287,786431,1310719,2147483647

%N Fixed points of A332221: Numbers k such that A156552(sigma(k)) is equal to k.

%C Equally, numbers k such that sigma(k) is equal to A005940(1+k).

%C The primes in this sequence are obtained by subtracting 1 from those terms of A029747 that are one more than a prime.

%C Questions: Are there other composite terms than 55 and 87? Are there other even terms than 2? (All such even terms should also occur in A332218).

%H <a href="/index/Bi#binary">Index entries for sequences related to binary expansion of n</a>

%H <a href="/index/Si#SIGMAN">Index entries for sequences related to sigma(n)</a>

%t Select[Range[10^5], DivisorSigma[1, #] == Block[{p = Partition[Split[Join[IntegerDigits[#, 2], {2}]], 2], q}, Times @@ Flatten[Table[q = Take[p, -i]; Prime[Count[Flatten[q], 0] + 1]^q[[1, 1]], {i, Length[p]}]]] &] (* _Michael De Vlieger_, Feb 12 2020, after _Robert G. Wilson v_ at A005940 *)

%Y Cf. A000203, A005940, A029747, A156552, A332217, A332218, A332221.

%Y Subsequences: A000668, A007505, A050522.

%K nonn,more

%O 1,1

%A _Antti Karttunen_, Feb 10 2020