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 A056853 #9 Sep 29 2019 02:37:26
%S A056853 4,6,7,12,13,15,18,19,21,30,42,45,60,63,72,93,102,108,117,138,150,165,
%T A056853 180,192,198,213,228,240,255,270,282,312,333,348,357,420,432,453,462,
%U A056853 522,525,570,600,618,642,660,693,717,765,810,822,828,858,882,933,957
%N A056853 Numbers n satisfying phi(n+1) - phi(n-1) = 2.
%H A056853 Amiram Eldar, <a href="/A056853/b056853.txt">Table of n, a(n) for n = 1..10000</a>
%e A056853 phi(13+1)-phi(13-1) = 2, so 13 is a term of the sequence.
%t A056853 Select[Range[10^3], EulerPhi[ # + 1] - EulerPhi[ # - 1] == 2 &]
%t A056853 Flatten[Position[Partition[EulerPhi[Range[1000]],3,1],_?(Last[#]-First[#] == 2&),{1},Heads->False]]+1 (* _Harvey P. Dale_, Jul 14 2014 *)
%Y A056853 Cf. A000010. Essentially the same as A001838.
%K A056853 nonn
%O A056853 1,1
%A A056853 _Joseph L. Pe_, Feb 12 2002