OFFSET
1,1
COMMENTS
In Euclidean geometry, a cyclic quadrilateral is a quadrilateral whose vertices all lie on a single circle. This circle is called the circumcircle or circumscribed circle, and the vertices are said to be concyclic.
The area A of a cyclic quadrilateral with sides a, b, c, d is given by Brahmagupta's formula: A = sqrt((s - a)(s - b)(s - c)(s - d)) where s, the semiperimeter is s = (a+b+c+d)/2.
The circumradius R (the radius of the circumcircle) is given by:
R = sqrt[(ab+cd)(ac+bd)(ad+bc)]/4A.
The corresponding R of a(n) are not unique; for example, for a(12) = 1680 => (a,b,c,d) = (24, 24, 70, 70) with R = 37 and (a,b,c,d) = (40, 40, 42,42) with R = 29.
The smallest corresponding R of a(n) is {5, 10, 13, 15, 17, 25, 20, 25, 26, 25, 41, 29, ...}.
Properties of this sequence:
A majority of quadrilaterals [a, b, c, d] have the property that a = b and c = d, and in this case s = a+c, A = a*c and R = sqrt(a^2+c^2)/2. Because a and c are even => a = 2p and c = 2q, then A = 4pq and R = sqrt(p^2+q^2). Consequently, 2*A103251(n) is included in this sequence.
Nevertheless, there also exist quadrilaterals whose four sides are distinct, for example [a, b, c, d] = [14, 30, 40, 48] => A = 936 = a(8) and R = 25. The subset of a(n) with this property is {936, 2856, 3744, 3864, 4536, 5016, 5376, 5712, 5880, 6696, 7056, 7560, ...}.
REFERENCES
Mohammad K. Azarian, Circumradius and Inradius, Problem S125, Math Horizons, Vol. 15, Issue 4, April 2008, p. 32.
LINKS
Mohammad K. Azarian, Solution to Problem S125: Circumradius and Inradius, Math Horizons, Vol. 16, Issue 2, November 2008, p. 32.
E. Gürel, Solution to Problem 1472, Maximal Area of Quadrilaterals, Math. Mag. 69 (1996), 149.
Eric Weisstein's World of Mathematics, Cyclic Quadrilateral
EXAMPLE
48 is in the sequence because, for (a,b,c,d) = (6,6,8,8),
s = (6+6+8+8)/2 = 14;
A = sqrt((14-6)(14-6)(14-8)(14-8)) = 48;
R = sqrt((6*6+8*8)(6*8+6*8)(6*8+6*8))/(4*48) = 960/192 = 5.
MATHEMATICA
SMax=8000;
Do[
Do[
x=S^2/(u v w);
If[u+v+w+x//OddQ, Continue[]];
If[v+w+x<=u, Continue[]];
r=Sqrt[v w+u x]Sqrt[u w+v x]Sqrt[u v+w x]/(4S);
If[r//IntegerQ//Not, Continue[]];
(*{a, b, c, d}=(u+v+w+x)/2-{u, v, w, x}; {a, b, c, d, r, S}//Sow*);
S//Sow; Break[]; (*to generate a table, comment out this line and uncomment previous line*)
, {u, S^2//Divisors//Select[#, S<=#^2&]&}
, {v, S^2/u//Divisors//Select[#, S^2<=u#^3&&#<=u&]&}
, {w, S^2/(u v)//Divisors//Select[#, S^2<=u v#^2&&#<=v&]&}
]
, {S, 24, SMax, 24}
]//Reap//Last//Last
{x, r, a, b, c, d}=.;
(* Albert Lau, May 25 2016 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Michel Lagneau, Mar 19 2012
EXTENSIONS
Incorrect Mathematica program removed by Albert Lau, May 25 2016
Missing term 5880 and more terms from Albert Lau, May 25 2016
STATUS
approved