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 A331862 #36 Feb 09 2020 03:01:03 %S A331862 3,26,186,206,258,3486,12602 %N A331862 Numbers n for which R(n) - 10^floor(n/2) is prime, where R(n) = (10^n-1)/9. %C A331862 The corresponding primes are a subsequence of A065074: near-repunit primes that contain the digit 0. %C A331862 In base 10, R(n) - 10^floor(n/2) has ceiling(n/2)-1 digits 1, one digit 0, and again floor(n/2) digits 1. For odd n, this is a palindrome, for even n the digit 0 is just left to the middle of the number. %C A331862 There can't be an odd term > 3 because the corresponding palindrome factors as R((n-1)/2)*(10^((n+1)/2) + 1). %C A331862 No term can be congruent to 1 mod 3. - _Chai Wah Wu_, Feb 07 2020 %H A331862 Brady Haran and Simon Pampena, <a href="https://www.youtube.com/watch?v=HPfAnX5blO0">Glitch Primes and Cyclops Numbers</a>, Numberphile video (2015) %e A331862 For n = 3, R(n) - 10^floor(n/2) = 101 is prime. %e A331862 For n = 26, R(n) - 10^floor(n/2) = 11111111111101111111111111 is prime. %o A331862 (PARI) for(n=0,9999,isprime(p=10^n\9-10^(n\2))&&print1(n",")) %Y A331862 Cf. A002275 (repunits), A004023 (indices of prime repunits), A011557 (powers of 10), A065074 (near-repunit primes that contain the digit 0), A105992 (near-repunit primes), A138148 (Cyclops numbers with digits 0 & 1). %Y A331862 Cf. A331860 (variant with digit 2 instead of digit 0), A331863 (variant with floor(n/2-1) instead of floor(n/2)). %K A331862 nonn,base,hard,more %O A331862 1,1 %A A331862 _M. F. Hasler_, Jan 30 2020 %E A331862 a(6)-a(7) from _Giovanni Resta_, Jan 31 2020