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 A065840 #14 Apr 07 2020 19:49:07
%S A065840 1,2,3,5,10,19,72,115,220,315,375,12408
%N A065840 Numbers n such that the first n quaternary digits found in the base-10 expansion of Pi form a prime (when the decimal point is ignored).
%C A065840 In other words, take the decimal expansion of Pi, drop any digits greater than 4, omit the decimal point and look for prefixes in the resulting string which form base-4 primes.
%C A065840 Numbers n such that A065838(n) is prime.
%C A065840 The next term in the sequence, if it exists, is greater than 10000. - _Nathaniel Johnston_, Nov 15 2010
%e A065840 E.g., the first a(5) or 10 quaternary digits of Pi are 31.12332323{4} and 3112332323{4} is the prime 880571{10}.
%t A065840 p = First[ RealDigits[ Pi, 10, 10^5]]; p = p[[ Select[ Range[10^5], p[[ # ]] == 0 || p[[ # ]] == 1 || p[[ # ]] == 2 || p[[ # ]] == 3 & ]]]; Do[ If[ PrimeQ[ FromDigits[ Take[p, n], 4]], Print[ n]], {n, 1, 4000} ]
%Y A065840 Cf. A065828 up to A065839, A000796, A011545, A011546, A055145, A005042, A060421, A039954, A048796.
%K A065840 nonn,base,hard
%O A065840 1,2
%A A065840 _Patrick De Geest_, Nov 24 2001
%E A065840 a(12) from _Chai Wah Wu_, Apr 07 2020