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 A280712 #16 Nov 09 2018 21:59:18
%S A280712 2,1,0,1,0,4,0,1,0,2,0,2,0,0,0,1,0,4,0,3,0,2,0,4,0,0,0,2,0,2,0,1,0,0,
%T A280712 0,4,0,0,0,4,0,4,0,1,0,2,0,1,0,0,0,2,0,2,0,1,0,2,0,7,0,0,0,1,0,2,0,0,
%U A280712 0,2,0,9,0,0,0,0,0,2,0,1,0,2,0,2,0,0,0,3,0,0,0,1,0,0,0,6,0,0,0,4,0,2,0,1,0
%N A280712 Inverse Euler transform of A280611.
%C A280712 a(n) = b(n) for n odd, a(n) = b(n) - b(n/2) for n even >= 2, where b(n) = A014197(n) = the number of m with phi(m) = n.
%C A280712 Note that a(n) = 0 for all odd n > 1, and so a(n) = b(n) for n >= 3, n not a multiple of 4.
%H A280712 Antti Karttunen, <a href="/A280712/b280712.txt">Table of n, a(n) for n = 1..20160</a>
%H A280712 Antti Karttunen, <a href="/A280712/a280712.txt">Data supplement: n, a(n) computed for n = 1..100000</a>
%F A280712 Euler transform of sequence = Product_{k>=1} (1-x^k)^(-a(k)) is the g.f. of A280611.
%e A280712 a(4) = #{m:phi(m) = 4} - #{m:phi(m) = 2} = #{5,8,10,12} - #{2,4,6} = 4-3 = 1.
%o A280712 (PARI)
%o A280712 A014197(n, m=1) = { n==1 && return(1+(m<2)); my(p, q); sumdiv(n, d, if( d>=m && isprime(d+1), sum( i=0, valuation(q=n\d, p=d+1), A014197(q\p^i, p))))}; \\ From A014197
%o A280712 A280712(n) = if(n%2,A014197(n),A014197(n)-A014197(n/2)); \\ _Antti Karttunen_, Nov 09 2018
%Y A280712 Cf. A014197, A280611.
%K A280712 easy,nonn
%O A280712 1,1
%A A280712 _Christopher J. Smyth_, Jan 07 2017
%E A280712 More terms from _Antti Karttunen_, Nov 09 2018