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 A218086 #51 Jan 20 2014 22:13:31
%S A218086 2,3,5,17,127
%N A218086 Mersenne prime exponents of prime index equal to 1 or another Mersenne prime exponent.
%C A218086 No others < 24036584 (see A000043, Mersenne exponents).
%C A218086 More formally: {n in N | 0 < d(2^n - 1) < 3 and 0 < d(2^Pi(n) - 1) < 3}; d(n) the divisor count function and Pi(n) the prime counting function.
%C A218086 To n = 4, this sequence = |A218386(n) - A215929(n)| = |{2, 5, 19, 257, 196687} - {0, 2, 24, 240, 196560}|
%C A218086 Conjecture: This sequence is complete.
%e A218086 Pi(2) = 1
%e A218086 Pi(3) = 2
%e A218086 Pi(5) = 3
%e A218086 Pi(17) = 7
%e A218086 Pi(127) = 31
%e A218086 {2, 3, 5, 17, 127} are Mersenne prime exponents.
%e A218086 {1, 2, 3, 7, 31} are Mersenne prime exponents at the beginning of the 20th century. (see A008578, noncomposite numbers)
%Y A218086 Cf. A218386, A215929
%K A218086 nonn
%O A218086 1,1
%A A218086 _Raphie Frank_, Oct 20 2012