A304973 Number of achiral color patterns (set partitions) for a row or loop of length n using exactly 3 colors (sets).

0, 0, 0, 1, 2, 5, 10, 19, 38, 65, 130, 211, 422, 665, 1330, 2059, 4118, 6305, 12610, 19171, 38342, 58025, 116050, 175099, 350198, 527345, 1054690, 1586131, 3172262, 4766585, 9533170, 14316139, 28632278, 42981185, 85962370, 129009091, 258018182, 387158345, 774316690, 1161737179, 2323474358

Offset: 0

Author

Robert A. Russell, May 22 2018

Keywords

• nonn
• easy

Comments

Two color patterns are equivalent if we permute the colors. Achiral color patterns must be equivalent if we reverse the order of the pattern.

Examples

			For a(5) = 5, the color patterns for both rows and loops are AABCC, ABACA, ABBBC, ABCAB, and ABCBA.
		

Links

• Index entries for linear recurrences with constant coefficients, signature (0, 5, 0, -6).

Crossrefs

Third column of A304972.

Third column of A140735 for odd n.

Third column of A293181 for even n.

Coefficients that determine the first formula and generating function are row 3 of A305008.

Programs

Mathematica

Table[If[EvenQ[n], 2 StirlingS2[n/2+1, 3] - 2 StirlingS2[n/2, 3], StirlingS2[(n + 3)/2, 3] - StirlingS2[(n + 1)/2, 3]], {n, 0, 30}]
Join[{0}, LinearRecurrence[{0, 5, 0, -6}, {0, 0, 1, 2}, 40]] (* Robert A. Russell, Oct 14 2018 *)

Formula

a(n) = [n==0 mod 2] * (2*S2(n/2+1, 3) - 2*S2(n/2, 3)) + [n==1 mod 2] * (S2((n+3)/2, 3) - S2((n+1)/2, 3)) where S2(n,k) is the Stirling subset number A008277(n,k).

G.f.: x^3 * (1+2x) / ((1-2x^2) * (1-3x^2)).

a(n) = A304972(n,3).

a(2m-1) = A140735(m,3).

a(2m) = A293181(m,3).