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 A189977 #15 Jan 13 2018 09:46:04 %S A189977 2,23,149,293,311,439,557,569,743,773,857,1031,1151,1493,1607,1663, %T A189977 1709,1733,1879,1913,2069,2141,2423,2711,2719,2729,2789,2969,3191, %U A189977 3209,3559,3607,3767,3821,3833,3847,3929,3967,4019,4073,4229,4339,4451,4517,4549 %N A189977 Primes p such that d(p+1) = 2*d(p-1), where d(k) counts the divisors of k. %H A189977 G. C. Greubel, <a href="/A189977/b189977.txt">Table of n, a(n) for n = 1..10000</a> %t A189977 Select[Prime[Range[PrimePi[5059]]], 2*DivisorSigma[0, # - 1] == DivisorSigma[0, # + 1] &] (* _T. D. Noe_, May 03 2011 *) %o A189977 (PARI) isok(p) = isprime(p) && (numdiv(p+1) == 2*numdiv(p-1)); \\ _Michel Marcus_, Jan 13 2018 %Y A189977 Cf. A000005, A008328, A008329. %K A189977 nonn %O A189977 1,1 %A A189977 _Ctibor O. Zizka_, May 03 2011