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 A259050 #21 May 03 2024 14:20:42 %S A259050 1,2,4,6,94,160,360,1470,2898,3094,3112,15698,17956,42262,111032, %T A259050 249550 %N A259050 Numbers k such that 3*R_k + 10^k - 2 is prime, where R_k = 11...11 is the repunit (A002275) of length k. %C A259050 Also, numbers k such that (4*10^k - 7)/3 is prime. %C A259050 Terms from Kamada data. %C A259050 a(16) > 2*10^5. %C A259050 The corresponding primes are a subset of the palindromes A185127 with a(n)+1 digits [1, 3 repeated a(n)-1 times, 1]: 11, 131, 13331, 1333331, ..., which can be expressed as 2*6-1, 2*66-1, 2*6666-1, 2*666666-1, ... . - _Hugo Pfoertner_, Jul 22 2020 %H A259050 Makoto Kamada, <a href="https://stdkmd.net/nrr/abbba.htm">Near-repdigit numbers of the form ABB...BBA</a>. %H A259050 Makoto Kamada, <a href="https://stdkmd.net/nrr/1/13331.htm#prime">Prime numbers of the form 133...331</a>. %H A259050 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>. %e A259050 For k=4, 3*R_4 + 10^k - 2 = 3333 + 10000 - 2 = 13331 which is prime. %t A259050 Select[Range[0, 200000], PrimeQ[(4*10^#-7)/3] &] %o A259050 (Magma) [n: n in [0..500] | IsPrime((4*10^n-7) div 3)]; // _Vincenzo Librandi_, Jun 18 2015 %o A259050 (PARI) for(k=1,1500,if(ispseudoprime(4*(10^k-1)/3-1),print1(k,", "))) \\ _Hugo Pfoertner_, Jul 22 2020 %Y A259050 Cf. A002275, A069882, A185127. %K A259050 nonn,hard,more %O A259050 1,2 %A A259050 _Robert Price_, Jun 17 2015 %E A259050 a(16) from Kamada data by _Tyler Busby_, May 03 2024