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 A213673 #34 Aug 22 2013 15:58:30 %S A213673 0,0,0,1,0,2,4,7,0,4,8,13,16,22,28,35,0,8,16,25,32,42,52,63,64,76,88, %T A213673 101,112,126,140,155,0,16,32,49,64,82,100,119,128,148,168,189,208,230, %U A213673 252,275,256,280,304,329,352,378,404,431,448,476,504 %N A213673 (n^2 - A000695(n))/4. %C A213673 A000695 is a 'pseudo-square function' that takes the binary expansion of n and moves bit k to bit 2k (that is, towards the most significant bits). %C A213673 a(n) = 0 if and only if n is 0 or a power of 2. %H A213673 R. J. Mathar, <a href="/A213673/b213673.txt">Table of n, a(n) for n = 0..1023</a> %e A213673 For example 7 = 111, and PSF(7) = 10101 = 21, so a(7) = (49 - 21)/4 = 7. %e A213673 13 = 1101 -> 1010001 = 81, so a(13) = (169 - 81)/4 = 22. %o A213673 (PARI) a(n)=my(v=binary(n));(n^2-sum(i=1,#v,v[i]*4^(#v-i)))/4 \\ _Charles R Greathouse IV_, Mar 04 2013 %Y A213673 Cf. A000290, A000695. %K A213673 nonn %O A213673 0,6 %A A213673 _Jon Perry_, Mar 03 2013