A082631 Enumeration of fractal 321 avoiders.

0, 1, 2, 6, 24, 116, 625, 3580, 21297, 130084, 810737, 5135142, 32961216, 213940962, 1401821021, 9260154974, 61603278951, 412352572116, 2775230479959, 18768654394542, 127482450424964, 869287197637752, 5948547336578541

Offset

0,3

Comments

See page 12-15 of link.

Links

Table of n, a(n) for n=0..22.

M. H. Albert, The fine structure of 321 avoiding permutations.

Formula

G.f. A(x) satisfies A(x)^6+(-2*x+3)*A(x)^4+(-2*x-1)*A(x)^3+(x^2-3*x+3)*A(x)^2+(2*x^2-2*x-1)*A(x)+x^2+x=0; a(n+1)/a(n) ->7.346751...

Prog

(PARI) a(n)=local(A); if(n<1, 0, A=x+O(x^2); for(m=2, n, A=Pol(A)+x*O(x^m); A=A^6+(-2*x+3)*A^4+(-2*x-1)*A^3+(x^2-3*x+3)*A^2+(2*x^2-2*x)*A+x^2+x); polcoeff(A, n))

Crossrefs

Sequence in context: A211321 A242820 A228395 * A212198 A182216 A097483

Adjacent sequences: A082628 A082629 A082630 * A082632 A082633 A082634

Keyword

nonn

Author

Benoit Cloitre, May 24 2003

Status

approved