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 A302049 #14 Dec 26 2018 02:36:43 %S A302049 0,0,0,0,0,1,0,0,0,0,0,0,0,0,1,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0, %T A302049 1,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0, %U A302049 0,0,0,0,0,0,0,0,1,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0 %N A302049 a(n) = 1 if n = prime(k)*prime(1+k) for some k, otherwise 0. %C A302049 Characteristic function for products of 2 successive primes (A006094). %H A302049 Antti Karttunen, <a href="/A302049/b302049.txt">Table of n, a(n) for n = 1..67591</a> %H A302049 <a href="/index/Ch#char_fns">Index entries for characteristic functions</a> %H A302049 <a href="/index/Pri#prime_indices">Index entries for sequences computed from indices in prime factorization</a> %F A302049 a(n) = A137794(n) * A280710(n). %F A302049 a(n) = A185013(A246277(n)) = A185013(A078898(n)). %o A302049 (PARI) A302049(n) = if(n<=1,0,my(p=precprime(sqrtint(n))); p>1 && 0==(n%p) && isprime(n/p) && (nextprime(p+1)==n/p)); \\ After code in A006094 %o A302049 (PARI) first(n) = my(res = vector(n), p = 2); forprime(q = 3, , if(p * q > n, return(res)); res[p * q]++; p = q) \\ _David A. Corneth_, Apr 24 2018 %Y A302049 Cf. A006094, A078898, A137794, A246277, A280710, A302047, A302048. %K A302049 nonn %O A302049 1,1 %A A302049 _Antti Karttunen_, Apr 24 2018