A035939 Number of partitions of n into parts not of the form 7k, 7k+3 or 7k-3. Also number of partitions such that the differences between parts at distance 2 are greater than 1.

1, 1, 2, 2, 3, 4, 6, 7, 10, 12, 16, 19, 25, 30, 38, 46, 57, 68, 84, 99, 121, 143, 172, 202, 242, 283, 336, 392, 462, 537, 630, 729, 851, 982, 1140, 1312, 1518, 1741, 2006, 2295, 2635, 3007, 3442, 3917, 4470, 5077, 5776, 6545, 7429, 8399, 9510, 10731

Offset

0,3

Comments

Case k=3, i=3 of Gordon Theorem.

Ramanujan theta functions: f(q) (see A121373), phi(q) (A000122), psi(q) (A010054), chi(q) (A000700).

References

G. E. Andrews, The Theory of Partitions, Addison-Wesley, 1976, p. 109.

Links

G. C. Greubel, Table of n, a(n) for n = 0..1000

Michael Somos, Introduction to Ramanujan theta functions

Eric Weisstein's World of Mathematics, Ramanujan Theta Functions

Formula

Expansion of f(-x^3, -x^4) / f(-x, -x^2) in powers of x where f() is Ramanujan's two-variable theta function. - Michael Somos, Dec 29 2014

Euler transform of period 7 sequence [ 1, 1, 0, 0, 1, 1, 0, ...].- Michael Somos, Dec 29 2014

G.f.: 1 / (Product_{k>0} (1 - x^(7*k - 6)) * (1 - x^(7*k - 5)) * (1 - x^(7*k - 2)) * (1 - x^(7*k - 1))). - Michael Somos, Dec 29 2014

G.f.: (Product_{k>0} (1 + x^k)) * (Sum_{k>=0} x^(2*k^2) / (Product_{i=1..k} (1 - x^(2*i)) * (1 + x^(2*i-1)) * (1 + x^(2*i)))). - Michael Somos, Dec 31 2014

a(n) ~ 2^(1/4) * cos(Pi/14) * exp(2*Pi*sqrt(2*n/21)) / (3^(1/4) * 7^(3/4) * n^(3/4)). - Vaclav Kotesovec, Nov 13 2015

Example

G.f. = 1 + x + 2*x^2 + 2*x^3 + 3*x^4 + 4*x^5 + 6*x^6 + 7*x^7 + 10*x^8 + ...
G.f. = q^-1 + q^41 + 2*q^83 + 2*q^125 + 3*q^167 + 4*q^209 + 6*q^251 + ...

Maple

# See A035937 for GordonsTheorem
A035939_list := n -> GordonsTheorem([1, 1, 0, 0, 1, 1, 0], n):
A035939_list(40); # Peter Luschny, Jan 22 2012

Mathematica

a[ n_] := SeriesCoefficient[ 1 / Product[ (1 - x^(7 k - 1)) (1 - x^(7 k - 2)) (1 - x^(7 k - 5)) (1 - x^(7 k - 6)), {k, Ceiling[n/7]}], {x, 0, n}]; (* Michael Somos, Dec 29 2014 *)
a[ n_] := SeriesCoefficient[ 1 / (QPochhammer[ x^1, x^7] QPochhammer[ x^2, x^7] QPochhammer[ x^5, x^7] QPochhammer[ x^6, x^7] ), {x, 0, n}]; (* Michael Somos, Dec 29 2014 *)

Prog

(Sage) # See A035937 for GordonsTheorem
def A035939_list(len) :  return GordonsTheorem([1, 1, 0, 0, 1, 1, 0], len)
A035939_list(40) # Peter Luschny, Jan 22 2012
(PARI) {a(n) = my(A); if( n<0, 0, polcoeff( 1 / prod( k=1, n, 1 -[0, 1, 1, 0, 0, 1, 1][k%7 + 1] * x^k, 1 + x * O(x^n)), n))}; /* Michael Somos, Dec 29 2014 */

Crossrefs

Cf. A035937, A035938.

Sequence in context: A353863 A102464 A082538 * A365827 A116665 A122135

Adjacent sequences: A035936 A035937 A035938 * A035940 A035941 A035942

Keyword

nonn,easy

Author

Olivier Gérard

Extensions

Missing a(0)=1 prepended by Michael Somos, Dec 29 2014

Name simplified by George Beck, Aug 27 2023

Status

approved