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 A281623 #19 Jul 26 2025 17:57:39 %S A281623 3,5,17,65,257,1025,4097,65537,262145,1048577,4194305,16777217, %T A281623 268435457,1073741825,4294967297,68719476737,1099511627777, %U A281623 4398046511105,17592186044417,70368744177665,281474976710657,4503599627370497,18014398509481985,72057594037927937 %N A281623 Numbers of the form 2^phi(m) + 1, where phi = A000010 = Euler totient function. %C A281623 Possible values of A243305 (2^phi(n) + 1). %C A281623 The first 5 known Fermat primes from A019434 are in the sequence. %e A281623 5 = 2^2 + 1 is a term because there are 3 numbers n (3, 4 and 6) with phi(n) = 2. %t A281623 Union[2^EulerPhi@ Range[10^3] + 1] (* _Michael De Vlieger_, Jan 30 2017 *) %o A281623 (Magma) Set(Sort([2^(EulerPhi(n)) + 1: n in[1..500]])); %Y A281623 Cf. A000010, A019434, A243305. %K A281623 nonn %O A281623 1,1 %A A281623 _Jaroslav Krizek_, Jan 25 2017