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 A130808 #6 Dec 24 2015 02:40:47 %S A130808 17,19,20,21,23 %N A130808 Magic side-sum constants of magic triangles of Yates using the numbers 1 to 9. %C A130808 A magic triangle of Yates, with nine numbers a,b,c,d,e,f,g,h,i has the form %C A130808 a %C A130808 d f %C A130808 e g %C A130808 b h i c %C A130808 The corner sum C = a+b+c and magic side-sum S = a+d+e+b = b+h+i+c = a+f+g+c are related by C=3*(S - 15). Since here, 1+2+3 <= C <= 7+8+9, we have 17 <= S <= 23. Finally, an argument based on a contradiction proof eliminates the values of 18 and 22 for S. %D A130808 U. Nagamani, "The magic triangles of Yates" in 'Science Reporter', pp. 65-66, Jan. 1978, Vol. 15, No. 1, CSIR, New Delhi. %e A130808 All the magic triangles are illustrated: %e A130808 1 1 4 3 7 %e A130808 9 6 8 9 8 1 8 7 3 6 %e A130808 5 7 6 2 3 9 4 2 5 1 %e A130808 2 8 4 3 4 5 3 7 5 7 2 6 6 5 1 9 8 2 4 9 %K A130808 fini,full,nonn %O A130808 1,1 %A A130808 _Lekraj Beedassy_, Aug 20 2007