A070109 - OEIS

%I #19 Oct 05 2021 10:59:10

%S 0,0,0,0,0,0,0,0,0,0,0,1,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,1,0,0,0,0,

%T 0,0,0,0,0,1,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,1,0,0,0,0,0,0,0,0,0,0,0,0,

%U 0,1,0,0,0,0,0,0,0,0,0,0,0,0,0,1,0,0,0,0,0,1,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0

%N Number of right integer triangles with perimeter n and relatively prime side lengths.

%C Right integer triangles have integer areas: see A070142, A051516.

%C a(n) is nonzero iff n is in A024364.

%H Antti Karttunen, <a href="/A070109/b070109.txt">Table of n, a(n) for n = 1..20000</a> (obtained from the b-file of A078926)

%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/RightTriangle.html">Right Triangle</a>.

%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/PythagoreanTriple.html">Pythagorean Triples</a>.

%H Reinhard Zumkeller, <a href="/A070080/a070080.txt">Integer-sided triangles</a>

%F a(n) = A078926(n/2) if n is even; a(n)=0 if n is odd.

%F a(n) = A051493(n) - A070094(n) - A070102(n).

%F a(n) <= A024155(n).

%e For n=30 there are A005044(30) = 19 integer triangles; only one is right: 5+12+13 = 30, 5^2+12^2 = 13^2; therefore a(30) = 1.

%t unitaryDivisors[n_] := Cases[Divisors[n], d_ /; GCD[d, n/d] == 1];

%t A078926[n_] := Count[unitaryDivisors[n], d_ /; OddQ[d] && Sqrt[n] < d < Sqrt[2n]];

%t a[n_] := If[EvenQ[n], A078926[n/2], 0];

%t Table[a[n], {n, 1, 1716}] (* _Jean-François Alcover_, Oct 04 2021 *)

%Y Cf. A070080, A070081, A070082, A051493, A070093, A070101, A070138, A070084, A070137.

%K nonn

%O 1,1716

%A _Reinhard Zumkeller_, May 05 2002

%E Secondary offset added by _Antti Karttunen_, Oct 07 2017