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 A306475 #21 Feb 21 2019 09:20:55 %S A306475 9,93,897,9569,31433,492170,4652430,47326803,436273150,4302407536, %T A306475 42652618575,738832928197,7177162612050,90874329411895, %U A306475 218209405436996,1693182318746937,80873624627235459,804212830686678390 %N A306475 Smallest nonprime number <= 10^n (n>=1) with maximum distance from a prime. %C A306475 Each number is a mean of two consecutive primes. %C A306475 Since, except 2, primes are odd numbers, this mean is an integer. %e A306475 For n=1: first prime numbers are 2, 3, 5, 7 and 11. Maximum difference between two consecutive primes is 4 between 7 and 11 thus a(1)=9. %e A306475 For n=4: maximum difference between two primes less than 10^4 is 36, which occurs once: between 9551 and 9587. a(4)=(9551 + 9587)/2 = 9569. %Y A306475 Cf. A000040, A001223, A087378, A282690. %K A306475 nonn,more %O A306475 1,1 %A A306475 _David Cobac_, Feb 18 2019 %E A306475 More terms (using the b-file at A002386) from _Jon E. Schoenfield_, Feb 19 2019