A376933 - OEIS

%I #14 Oct 14 2024 00:10:01

%S 2,3,9,219,89,707,7139,17397,127107,185573,170099,8513673,3730629,

%T 9645947,20878187,435128997,413502597,7180622897,28071202809,

%U 4197184407,53159437779,72827487477,408466487673,1622948986427,1009480191957,50924645281527,141362538039039

%N a(n) = (A376907(n) - 10^(n-1))/3.

%t a[n_]:=(Module[{k=1}, While[!PrimeQ[m=3k^2+3k+1]||IntegerLength[m]<n, k++]; m]-10^(n-1))/3; Array[a, 15]

%o (Python)

%o from itertools import count

%o from math import isqrt

%o from sympy import isprime

%o def A376933(n):

%o     for k in count(isqrt((((a:=10**(n-1))<<2)-1)//12)):

%o         m = 3*k*(k+1)+1

%o         if m >= a and isprime(m):

%o             return (m-a)//3 # _Chai Wah Wu_, Oct 13 2024

%Y Cf. A011557, A376907.

%K nonn,base,more

%O 1,1

%A _Stefano Spezia_, Oct 11 2024

%E a(21)-a(27) from _Chai Wah Wu_, Oct 13 2024