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 A111406 #13 Dec 07 2017 09:48:35 %S A111406 0,0,1,0,1,0,0,0,0,0,1,0,0,0,0,0,1,0,0,0,0,0,0,0,0,0,0,0,0,0,1,0,0,0, %T A111406 0,0,0,0,0,0,1,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,1,0,0,0,0,0,0,0,1,0, %U A111406 0,0,0,0,0,0,0,0,0,0,0,0,0,0,1,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0 %N A111406 a(n) = f(f(n+1)) - f(f(n)), where f(m) = pi(m) = A000720(m), with f(0) = 0. %C A111406 Right-shifted sequence is characteristic function of prime-indexed primes (A006450). - _Franklin T. Adams-Watters_, Nov 17 2006 %H A111406 Antti Karttunen, <a href="/A111406/b111406.txt">Table of n, a(n) for n = 0..65557</a> %H A111406 <a href="/index/Ch#char_fns">Index entries for characteristic functions</a> %o A111406 (PARI) A111406(n) = (primepi(primepi(n+1)) - primepi(primepi(n))); \\ _Antti Karttunen_, Dec 07 2017 %Y A111406 Cf. A000720, A006450. %K A111406 nonn %O A111406 0,1 %A A111406 _N. J. A. Sloane_, Nov 12 2005 %E A111406 Missing right parenthesis added to the formula and case f(0) clarified by _Antti Karttunen_, Dec 07 2017