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 A371224 #15 Mar 29 2025 19:45:24
%S A371224 0,0,2,3,5,0,2,2,3,3,5,2,13,163,43,13,2,317213509,2,2,2,2,2,2,83,41,2,
%T A371224 3,2,3,3,5,2,2,2,2,2,0,13,59,3,2,3,3,3,3,3,31,3,1657,2,3,2,2,2,29,13,
%U A371224 2,3,2,2,5,2828293681646068747,2,3,2,223,2,7
%N A371224 Least prime factor of the integer formed by the first n decimal digits of Pi, or 0 if that number is prime.
%C A371224 Complementary to the sequences A005042 (primes in the initial digits of Pi) and A060421 which lists numbers N such that the first N digits of Pi form a prime - exactly the indices of zeros in the present sequence.
%F A371224 a(n) = 0 <=> n is in A060421 <=> A011545(n-1) is in A000040 (primes).
%F A371224 a(n) = A089281(n) = A020639(A011545(n-1)) whenever a(n) is nonzero.
%F A371224 a(n) = 2 <=> A000796(2-n) is even <=> A011545(n-1) is even.
%o A371224 (PARI) a(n, c=Pi)={ if( n>=precision(c), error("insufficient precision"), !ispseudoprime(c\10^-n--), factor(c\.1^n)[1,1], 0)}
%Y A371224 Cf. A000796 (decimals of Pi), A005042 (primes in A011545), A011545 (integer made of n+1 initial digits of Pi), A060421 (length of A005042(n)), A020639 (smallest prime factor of n), A000040 (primes), A089281 (smallest prime factor of A011545(n)).
%K A371224 nonn,base
%O A371224 1,3
%A A371224 _M. F. Hasler_, Mar 15 2024