A386753 Decimal expansion of the surface area of a bilunabirotunda with unit edges.

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

Offset: 2

Author

Paolo Xausa, Aug 01 2025

Keywords

• nonn
• cons
• easy

Comments

The bilunabirotunda is Johnson solid J_91.

Examples

			12.346011217493622278090940592566183131515107...
		

Links

• Paolo Xausa, Table of n, a(n) for n = 2..10000
• Wikipedia, Bilunabirotunda.

Crossrefs

Cf. A189963 (volume - 1).

Cf. A010469, A010476.

Programs

Mathematica

First[RealDigits[2 + Sqrt[12] + Sqrt[5*(5 + Sqrt[20])], 10, 100]] (* or *)
First[RealDigits[PolyhedronData["J91", "SurfaceArea"], 10, 100]]

Formula

Equals 2 + 2*sqrt(3) + sqrt(5*(5 + 2*sqrt(5))) = 2 + A010469 + sqrt(5*(5 + A010476)).

Equals the largest root of x^8 - 16*x^7 - 36*x^6 + 1328*x^5 - 2946*x^4 - 16624*x^3 + 34796*x^2 + 61392*x - 10679.