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A228465 Recurrence a(n) = a(n-1) + 2^n*a(n-2) with a(0)=0, a(1)=1. 3
0, 1, 1, 9, 25, 313, 1913, 41977, 531705, 22023929, 566489849, 45671496441, 2366013917945, 376506912762617, 39141278944373497, 12376519796349807353, 2577539376694811306745, 1624792742123856760679161, 677311275106408471956040441, 852536648457739021814912002809 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,4

COMMENTS

Generally (if p>0, q>1), recurrence a(n) = b*a(n-1) + (p*q^n+d)*a(n-2), a(n) is asymptotic to c*q^(n^2/4)*(p*q)^(n/2), where c is for fixed parameters b, p, d, q, a(0), a(1) constant, independent on n.

LINKS

Vincenzo Librandi, Table of n, a(n) for n = 0..100

FORMULA

a(n) ~ c * 2^(n^2/4 + n/2), where c = 0.548441579870783378573455400152590154... if n is even and c = 0.800417244834941368929416800341853541... if n is odd.

a(n) = Sum_{k=1..floor(n/2+1/2)} qbinomial(n-k,k-1)*2^(k^2-1), where q-binomial is triangle A022166, that is, with q=2. - Vladimir Kruchinin, Jan 20 2020

MATHEMATICA

RecurrenceTable[{a[n]==a[n-1]+2^n*a[n-2], a[0]==0, a[1]==1}, a, {n, 0, 20}]

(* Alternative: *)

a[n_] := Sum[2^(k^2-1) QBinomial[n - k , k - 1, 2], {k, 1, n}];

Table[a[n], {n, 0, 19}] (* After Vladimir Kruchinin. Peter Luschny, Jan 20 2020 *)

PROG

(MAGMA) [n le 2 select (n-1) else Self(n-1)+Self(n-2)*2^(n-1): n in [1..20]]; // Vincenzo Librandi, Aug 23 2013

(SageMath)

def a(n):

    return sum(2^(k^2 - 1)*q_binomial(n-k , k-1, 2) for k in (1..n))

print([a(n) for n in range(20)]) # Peter Luschny, Jan 20 2020

CROSSREFS

Cf. A006116, A006117, A006118, A006119, A006120, A006121, A006122.

Sequence in context: A278911 A330828 A201807 * A048056 A038534 A117755

Adjacent sequences:  A228462 A228463 A228464 * A228466 A228467 A228468

KEYWORD

nonn

AUTHOR

Vaclav Kotesovec, Aug 22 2013

STATUS

approved

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Last modified July 14 16:21 EDT 2020. Contains 335729 sequences. (Running on oeis4.)