A340859 - OEIS

%I #9 Feb 22 2021 02:25:40

%S 0,0,0,1,1,1,2,5,6,3,3,9,6,5,10,20,9,10,8,21,18,10,10,37,21,12,24,31,

%T 14,26,17,55,32,20,36,54,22,20,39,74,24,40,26,58,59,24,26,113,47,41,

%U 54,69,33,51,61,111,65,35,39,124,38,39,88,145,79

%N a(n) is the number of isosceles integer trapezoids (up to congruence) with integer side lengths a,c,b=d with n=Max(a,b,c) and integer diagonals e=f.

%C By "trapezoid" here is meant a quadrilateral with exactly one pair of parallel sides.

%C Without loss of generality we assume b=d and for the parallel sides c < a. e and f are uniquely determined by e = f = sqrt((c(a^2-b^2) + a(b^2-c^2))/(a-c)). The smallest possible isosceles trapezoid has side lengths a=4, c=3, b=d=2 and diagonals e=f=4.

%e a(7)=2 because there are two possible trapezoids: a=5, c=3, b=d=7, e=f=8 and a=7, c=4, b=d=6, e=f=8.

%t n=65;list={};

%t For[a=1,a<=n,a++,

%t For[c=1,c<a,c++,

%t For[d=Floor[(a-c)/2]+1,d<=n,d++,

%t For[b=1,b<=n,b++,

%t se=c(a^2-b^2)+a(d^2-c^2);sf=c(a^2-d^2)+a(b^2-c^2);

%t If[se<=0||sf>se,Break[]];If[sf<=0,Continue[]];

%t e=Sqrt[se/(a-c)];f=Sqrt[sf/(a-c)];

%t If[IntegerQ[e]&&IntegerQ[f]&&a+d>f&&d+f>a&&f+a>d&&e+b>a&&b+a>e&&a+e>b,AppendTo[list,{a,b,c,d,e,f}]]]]]]

%t Table[Select[list,Max[#[[1]],#[[2]],#[[3]],#[[4]]]==n&&#[[2]]==#[[4]]&]//Length,{n,1,65}]

%Y Cf. A224931 for parallelograms, A340858 for general trapezoids and A340860 for non-isosceles trapezoids.

%K nonn

%O 1,7

%A _Herbert Kociemba_, Jan 24 2021