A309404 - OEIS

%I #9 Jun 18 2020 14:00:58

%S 6,7,8,9,11,12,13,15,16,17,19,21,24,25,27,28,29,31,33,35,36,37,39,40,

%T 41,43,45,47,48,49,51,53,55,57,59,60,61,63,64,65,67,69,71,73,75,76,77,

%U 79,81,83,84,85,87,88,89,91,93,95,96,97,99,101,103,105,107

%N Perimeters of integer-sided isosceles triangles with prime side lengths.

%H Wikipedia, <a href="https://en.wikipedia.org/wiki/Integer_triangle">Integer Triangle</a>

%t Flatten[Table[If[Sum[Sum[(PrimePi[i] - PrimePi[i - 1]) (PrimePi[k] - PrimePi[k - 1]) (PrimePi[n - i - k] - PrimePi[n - i - k - 1]) (KroneckerDelta[i, k] + KroneckerDelta[i, n - i - k] - KroneckerDelta[k, n - i - k]) Sign[Floor[(i + k)/(n - i - k + 1)]], {i, k, Floor[(n - k)/2]}], {k, Floor[n/3]}] > 0, n, {}], {n, 100}]]

%Y Cf. A308147.

%K nonn

%O 1,1

%A _Wesley Ivan Hurt_, Jul 28 2019