A272390 Longest side of primitive Heronian tetrahedron with 4 congruent triangle faces.

203, 888, 1804, 2431, 2873

Offset

1,1

Comments

A Heronian tetrahedron or perfect tetrahedron is a tetrahedron whose edge lengths, face areas and volume are all integers.

Primitive tetrahedron means 4 edge lengths share no common factor.

Properties:

1. 3 pairs of opposite edge lengths are equal.

2. The perimeter must be an even number.

3. The faces are acute triangles, and cannot be isosceles triangle.

It is known that 5512,8484,11275,19695,32708,294175,683787 are in the sequence.

Links

Table of n, a(n) for n=1..5.

Example

Below shows some example: (might contains gap)
a,     b,     c,     S,         V
203,   195,   148,   13650,     611520
888,   875,   533,   223860,    37608480
1804,  1479,  1183,  870870,    214582368
2431,  2296,  2175,  2277660,   1403038560
2873,  2748,  1825,  2419950,   1355172000
5512,  5215,  1887,  4919460,   1377448800
8484,  6625,  6409,  20980050,  30546952800
11275, 10136, 8619,  41861820,  103147524480
19695, 16448, 13073, 106675680, 323290060800
32708, 31493, 24525, 363332970, 2685757314240

Mathematica

heron=1/4Sqrt[(#1+#2+#3)(-#1+#2+#3)(#1-#2+#3)(#1+#2-#3)]&;
cayley=1/24Sqrt[2Det[{
  {0, 1, 1, 1, 1},
  {1, 0, #1^2, #2^2, #6^2},
  {1, #1^2, 0, #3^2, #5^2},
  {1, #2^2, #3^2, 0, #4^2},
  {1, #6^2, #5^2, #4^2, 0}
}]]&;
aMin=203;
aMax=2000(*WARNING:runs very slow*);
Do[
  If[GCD[a, b, c]>1, Continue[]];
  S=heron[a, b, c];
  If[S//IntegerQ//Not, Continue[]];
  V=cayley[a, b, c, a, b, c];
  If[V//IntegerQ//Not, Continue[]];
  a(*{a, b, c, S, V}*)//Sow;
  , {a, aMin, aMax}
  , {b, a/Sqrt[2]//Ceiling, a-1}
  , {c, Mod[a+b, 2, Floor[Sqrt[a^2-b^2]]+1], b-1, 2}
]//Reap//Last//Last(*//TableForm*)
{S, V}=.;
(*
(*this piece of code runs much faster but might contains gap*)
mMax=100;
Do[
  {a, b, c}={n(m^2+k^2), m(n^2+k^2), (m+n)(m n-k^2)};
  {a, b, c}={a, b, c}/GCD[a, b, c];
  V=cayley[a, b, c, a, b, c];
  If[V//IntegerQ//Not, Continue[]];
  a(*{a, b, c, heron[a, b, c], V}*)//Sow
  , {m, mMax}
  , {n, m-1}
  , {k, Floor[Sqrt[(m^2 n)/(2m+n)]+1], n-1}
]//Reap//Last//Last//Union(*TableForm*)
{a, b, c, V}=.;
*)

Crossrefs

Sequence in context: A211565 A346826 A346859 * A320557 A270770 A294056

Adjacent sequences: A272387 A272388 A272389 * A272391 A272392 A272393

Keyword

nonn,more

Author

Albert Lau, May 26 2016

Status

approved