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 A322985 #18 Mar 19 2023 02:31:20 %S A322985 1,5,17,23,25,28,91,187,287,398,899,1364,2921,5125,5890,8780,14881, %T A322985 35689,46669,71861,111710 %N A322985 Numbers k such that 123456789*10^k+1 is prime. %C A322985 a(22) > 1.3*10^5. All numbers up to this bound were sieved using newpgen and sr1sieve. Remaining numbers were checked for primality using Jean Penné's LLR application (BLS (N-1/N+1) test). %e A322985 1 is a term because 1234567891 is prime. %e A322985 2 is not a term because 12345678901 is composite (it is divisible by 857). %t A322985 Select[Range@ 1400, PrimeQ[123456789*10^# + 1] &] (* _Michael De Vlieger_, Jan 04 2019 *) %o A322985 (Python) %o A322985 from sympy.ntheory.primetest import isprime %o A322985 for n in range(1,1000): %o A322985 if isprime(123456789*10**n+1): %o A322985 print(n, end=', ') # _Stefano Spezia_, Jan 05 2019 %Y A322985 Cf. A248349, A248350, A321806. %K A322985 nonn,more %O A322985 1,2 %A A322985 _Matthias Baur_, Jan 01 2019