A387989 Decimal expansion of the perimeter equal to the area of an equable decagon (10-gon).

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

Offset

2,2

Comments

The smallest positive solution to x^4 - 3200 * x^2 + 512000 = 0.

Links

Table of n, a(n) for n=2..106.

Eric Weisstein's World of Mathematics, Regular Decagon.

Wikipedia, Decagon.

Wikipedia, Equable shape.

Index entries for algebraic numbers, degree 4.

Formula

Equals 40 * sqrt(1 - 2 / sqrt(5)).

Equals 40 * sqrt(1 - 2 / A002163).

Equals 40 * sqrt(1 - A010532 / 10).

Equals 8 * sqrt(25 - 10 * sqrt(5)).

Equals 40 * tan(Pi/10).

Equals 40 * A019916.

Equals 40 * A019827 / A019881.

Equals 40 / A019970.

Equals 100 / A178816.

Example

12.9967878493162530...

Maple

evalf(40 * sqrt(1 - 2 / sqrt(5)), 100);

Mathematica

RealDigits[40 * Sqrt[1 - 2/Sqrt[5]], 10, 100][[1]]

Prog

(PARI) 40 * sqrt(1 - 2 / sqrt(5))

Crossrefs

Cf. A002163, A010532, A019827, A019881, A019916, A019970, A178816.

Sequence in context: A305308 A087042 A266274 * A390428 A003678 A201683

Adjacent sequences: A387986 A387987 A387988 * A387990 A387991 A387992

Keyword

nonn,cons

Author

Michal Paulovic, Oct 13 2025

Status

approved