1,2

Two decks each have n kinds of cards, 2 of each kind. The first deck is laid out in order. The second deck is shuffled and laid out next to the first. A match occurs if a card from the second deck is next to a card of the same kind from the first deck. A(n) is the number of ways of achieving no matches. The probability of no matches is a(n)/(2n)!.

n couples meet for a party and they exchange gifts. Each of the 2n writes their name on a piece of paper and puts it into a hat. They take turns drawing names and give their gift to the person whose name they drew. If anyone draws either their own name or the name of their partner, everyone puts the name they have drawn back into the hat and everyone draws anew. a(n) is the number of different permissible drawings. - Dan Dima (dimad72(AT)gmail.com), Dec 17 2006

(2n)! / a(n) is the expected number of deck shuffles until no matches occur. a(n) / (2n)! is the probability for a permissible drawing to be achieved. (2n)! / a(n) is the expected number of drawings before a permissible drawing is achieved. As n goes to infinity (2n)! / a(n) will strictly decrease very slowly to e^2 ~ 7.38906 (starting from n > 2) - Dan Dima (dimad72(AT)gmail.com), Dec 17 2006

N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).

F. N. David and D. E. Barton, Combinatorial Chance, Hafner, NY, 1962, Ch. 7 and Ch. 12.

F. F. Knudsen and I. Skau, On the Asymptotic Solution of a Card-Matching Problem, Mathematics Magazine 69 (1996), 190-197.

B. H. Margolius, The Dinner-Diner Matching Problem, Mathematics Magazine, 76 (2003), 107-118.

J. Riordan, An Introduction to Combinatorial Analysis, Wiley, 1958, p. 187.

T. D. Noe, Table of n, a(n) for n=1..100

Barbara H. Margolius, Dinner-Diner Matching Probabilities

Index entries for sequences related to card matching

G.f.: sum(coeff(R(x, n, k), x, j)*(t-1)^j*(n*k-j)!, j=0..n*k) where n is the number of kinds of cards, k is the number of cards of each kind (2 in this case) and R(x, n, k) is the rook polynomial given by R(x, n, k)=(k!^2*sum(x^j/((k-j)!^2*j!))^n (see Riordan). coeff(R(x, n, k), x, j) indicates the coefficient for x^j of the rook polynomial.

a(n) = n! * sum((-1)^b * 2^{a+2b} * (2n-2a-b)! / (a! * b! * (n-a-b)!), a,b >= 0, a+b <= n) a(n) = n * a(n-1) + n! * 4^n * sum((-1)^a / (a! * 2^a), a=0 to n) - Dan Dima (dimad72(AT)gmail.com), Dec 17 2006

There are 80 ways of achieving zero matches when there are 2 cards of each kind and 3 kinds of card so A(3)=80.

p := (x, k)->k!^2*sum(x^j/((k-j)!^2*j!), j=0..k); R := (x, n, k)->p(x, k)^n; f := (t, n, k)->sum(coeff(R(x, n, k), x, j)*(t-1)^j*(n*k-j)!, j=0..n*k); seq(f(0, n, 2), n=0..18);

2^n*A000459.

Sequence in context: A013007 A013008 A013180 this_sequence A012106 A156103 A116189

Adjacent sequences: A000313 A000314 A000315 this_sequence A000317 A000318 A000319

nonn,easy,nice

N. J. A. Sloane (njas(AT)research.att.com).

Formulae, more terms etc. from Barbara Haas Margolius (margolius(AT)math.csuohio.edu), Dec 22 2000