A199861 The decimal expansion (unsigned) of the value of d that maximizes the Brahmagupta expression given below.

2, 2, 7, 1, 0, 6, 4, 4, 8, 2, 9, 4, 3, 8, 1, 2, 0, 3, 0, 1, 1, 1, 4, 3, 3, 5, 2, 5, 3, 2, 3, 4, 4, 6, 1, 8, 3, 7, 7, 5, 4, 0, 5, 3, 1, 2, 9, 8, 6, 7, 4, 9, 6, 2, 9, 3, 2, 5, 4, 0, 3, 5, 4, 5, 5, 0, 4, 8, 1, 2, 6, 1, 0, 0, 0, 1, 6, 0, 1, 8, 4, 3, 7, 1, 1, 6, 7, 7, 4, 5, 2, 8, 4, 9, 4, 9, 4, 5, 8, 6, 3, 5, 8

Offset: 0

Author

Frank M Jackson, Nov 11 2011

Keywords

• nonn
• cons

Comments

Brahmagupta expression sqrt((-1+1/(1+d)+1/(1+2d)+1/(1+3d)) * (1-1/(1+d)+1/(1+2d)+1/(1+3d)) * (1+1/(1+d)-1/(1+2d)+1/(1+3d)) * (1+1/(1+d)+1/(1+2d)-1/(1+3d)))/4 for d in the interval [-1/3, inf] where 1/(1+d), 1/(1+2d) and 1/(1+3d) are always positive.

The area of a convex quadrilateral with fixed sides is maximal when it is organized as a convex cyclic quadrilateral. Furthermore in order that a quadrilateral can have sides in a harmonic progression 1 : 1/(1+d) : 1/(1+2d) : 1/(1+3d) its denominator's common difference d is limited to the range f < d < g where f is the constant A199590 and g is the constant A199589. Consequently when d=-0.2271064482... it maximizes Brahmagupta's expression for the area of a convex cyclic quadrilateral whose sides form a harmonic progression.

Examples

			-0.22710644829438120301114335253234461837754...
		

Links

• Wikipedia, Brahmagupta's formula.
• Index entries for algebraic numbers, degree 12

Crossrefs

Cf. A010503, A193180, A193211, A199589, A199590.

Programs

Mathematica

RealDigits[d/.NMaximize[{Sqrt[(-1+1/(1+d)+1/(1+2d)+1/(1+3d))(1-1/(1+d)+1/(1+2d)+1/(1+3d))(1+1/(1+d)-1/(1+2d)+1/(1+3d))(1+1/(1+d)+1/(1+2d)-1/(1+3d))]/4, -1/4120, PrecisionGoal->100, WorkingPrecision->240][[2]]][[1]]

PARI

real(polroots(1323*d^12 + 9711*d^11 + 32535*d^10 + 67005*d^9 + 94338*d^8 + 94761*d^7 + 68955*d^6 + 36367*d^5 + 13740*d^4 + 3619*d^3 + 630*d^2 + 65*d + 3)[4]) \\ Charles R Greathouse IV, Nov 11 2011

PARI

polrootsreal(1323*x^12 - 9711*x^11 + 32535*x^10 - 67005*x^9 + 94338*x^8 - 94761*x^7 + 68955*x^6 - 36367*x^5 + 13740*x^4 - 3619*x^3 + 630*x^2 - 65*x + 3)[1] \\ Charles R Greathouse IV, Oct 27 2023

Formula

d is the largest real root of the equation 1323d^12 + 9711d^11 + 32535d^10 + 67005d^9 + 94338d^8 + 94761d^7 + 68955d^6 + 36367d^5 + 13740d^4 + 3619d^3 + 630d^2 + 65d + 3 = 0.