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 A076647 #11 Aug 15 2019 06:47:54 %S A076647 56,456,476,828,846,1196,2060,2780,4484,7530,9174,11420,11475,11865, %T A076647 13454,22154,34916,35385,52185,67308,105798,140895,168872,190840, %U A076647 219852,241892,244074,267225,303834,317564,330484,335440,460292,580970,658515 %N A076647 Numbers n such that phi(n) + phi(n+1) = sigma(n)/2. %H A076647 Donovan Johnson, <a href="/A076647/b076647.txt">Table of n, a(n) for n = 1..200</a> %e A076647 phi(56) + phi(57) = 24 + 36 = 60 = 1/2 sigma(56), so 56 is a term of the sequence. %p A076647 with(numtheory): p:=proc(n) if phi(n)+phi(n+1)=sigma(n)/2 then n else fi end: seq(p(n),n=1..2500000); # _Emeric Deutsch_, Sep 02 2005 %t A076647 Select[Range[10^5], EulerPhi[ # ] + EulerPhi[ # + 1] == (1/2)DivisorSigma[1, # ] &] %o A076647 (PARI) isok(n) = eulerphi(n) + eulerphi(n+1) == sigma(n)/2; \\ _Michel Marcus_, Aug 15 2019 %Y A076647 Cf. A092404. %K A076647 nonn %O A076647 1,1 %A A076647 _Joseph L. Pe_, Oct 23 2002 %E A076647 More terms from _Emeric Deutsch_, Sep 02 2005