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Number of parking functions of size n avoiding the patterns 213 and 312.
5

%I #16 Jan 11 2024 09:12:56

%S 1,1,3,12,54,259,1293,6634,34716,184389,990711,5372088,29347794,

%T 161317671,891313569,4946324886,27552980088,153982124809,862997075691,

%U 4848839608228,27304369787694,154059320699211,870796075968693,4929937918315522,27950989413184404

%N Number of parking functions of size n avoiding the patterns 213 and 312.

%H Ayomikun Adeniran and Lara Pudwell, <a href="https://doi.org/10.54550/ECA2023V3S3R17">Pattern avoidance in parking functions</a>, Enumer. Comb. Appl. 3:3 (2023), Article S2R17.

%F For n>=1, a(n) = Sum_{k=0..n-1} Sum_{i=0..k} binomial(n - 1, i)*(k + 1)*binomial(2*n - 2 - k, n - 1 - k)/n.

%F D-finite with recurrence (n+1)*a(n) +3*(-4*n+1)*a(n-1) +(34*n-45)*a(n-2) +3*(4*n-17)*a(n-3) +3*(-n+4)*a(n-4)=0. - _R. J. Mathar_, Jan 11 2024

%e For n=3 the a(3)=12 parking functions, given in block notation, are {1},{2},{3}; {1,2},{},{3}; {1,2},{3},{}; {1},{2,3},{}; {1,2,3},{},{}; {1},{3},{2}; {1,3},{},{2}; {1,3},{2},{}; {2},{3},{1}; {2,3},{},{1}; {2,3},{1},{}; {3},{2},{1}.

%p A362597 := proc(n)

%p if n = 0 then

%p 1;

%p else

%p add(add(binomial(n - 1, i)*(k + 1)*binomial(2*n - 2 - k, n - 1 - k)/n,i=0..k),k=0..n-1) ;

%p end if;

%p end proc:

%p seq(A362597(n),n=0..60) ; # _R. J. Mathar_, Jan 11 2024

%o (PARI) a(n)={0^n + sum(k=0, n-1, sum(i=0, k, binomial(n - 1, i)*(k + 1)*binomial(2*n - 2 - k, n - 1 - k)/n))} \\ _Andrew Howroyd_, Apr 27 2023

%Y Cf. A028365, A362596.

%K nonn,easy

%O 0,3

%A _Lara Pudwell_, Apr 27 2023