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 A057776 #17 Mar 16 2025 13:11:23 %S A057776 1,2,3,13,7,25,44,116,55,974,1581,2111,1470,4289,10847,15000,6543, %T A057776 91466,62947,397907,498178,1452314,6025010,20197904,38946356,9385401, %U A057776 24843812,98842359,166808880,556542914,154570517,3132108468,7417604438,3217817383,47999122016 %N A057776 a(n) is the least number k such that prime(k) - 1 is divisible by 2^(n-1) and the quotient is odd. %H A057776 Amiram Eldar, <a href="/A057776/b057776.txt">Table of n, a(n) for n = 1..72</a> %F A057776 a(n) = PrimePi(A057775(n-1)). - _Amiram Eldar_, Mar 16 2025 %e A057776 For n = 1, a(1) = 1, prime(a(1)) = prime(1) = 2 and prime(1)-1 = 1 is divisible by 2^(n-1) = 2^0 = 1; moreover 2 is the smallest. %e A057776 For n = 10, a(10) = 974, the 974th prime is 7681, prime(974) - 1 = 7680 = 512*15, is divisible by 2^9 = 512 and the quotient is 15, and there are no other primes such this below 7681. %e A057776 A057775(30) = 12348030977; a(30) = 556542914. It means that 12348030977 is the 556542914th prime. A057777(30) = 12348030976; when A057777(30) is divided by 2^29, the quotient is 23 = A057778(30). %Y A057776 Cf. A000040, A000720, A006093, A057773, A057775, A057776, A057777, A057778. %K A057776 nonn %O A057776 1,2 %A A057776 _Labos Elemer_, Nov 02 2000 %E A057776 a(32)-a(35) from _Amiram Eldar_, Mar 16 2025