A319751 Number of non-isomorphic set systems of weight n with empty intersection.

1, 0, 1, 2, 6, 13, 35, 83, 217, 556, 1504, 4103, 11715, 34137, 103155, 320217, 1025757, 3376889, 11436712, 39758152, 141817521, 518322115, 1939518461, 7422543892, 29028055198, 115908161428, 472185530376, 1961087909565, 8298093611774, 35750704171225, 156734314212418

Offset

0,4

Comments

A set system is a finite set of finite nonempty sets. Its weight is the sum of sizes of its parts. Weight is generally not the same as number of vertices.

Links

Andrew Howroyd, Table of n, a(n) for n = 0..50

Example

Non-isomorphic representatives of the a(2) = 1 through a(5) = 13 set systems:
2: {{1},{2}}
3: {{1},{2,3}}
   {{1},{2},{3}}
4: {{1},{2,3,4}}
   {{1,2},{3,4}}
   {{1},{2},{1,2}}
   {{1},{2},{3,4}}
   {{1},{3},{2,3}}
   {{1},{2},{3},{4}}
5: {{1},{2,3,4,5}}
   {{1,2},{3,4,5}}
   {{1},{2},{3,4,5}}
   {{1},{4},{2,3,4}}
   {{1},{2,3},{4,5}}
   {{1},{2,4},{3,4}}
   {{2},{3},{1,2,3}}
   {{2},{1,3},{2,3}}
   {{4},{1,2},{3,4}}
   {{1},{2},{3},{2,3}}
   {{1},{2},{3},{4,5}}
   {{1},{2},{4},{3,4}}
   {{1},{2},{3},{4},{5}}

Prog

(PARI)
WeighT(v)={Vec(exp(x*Ser(dirmul(v, vector(#v, n, (-1)^(n-1)/n))))-1, -#v)}
permcount(v) = {my(m=1, s=0, k=0, t); for(i=1, #v, t=v[i]; k=if(i>1&&t==v[i-1], k+1, 1); m*=t*k; s+=t); s!/m}
K(q, t, k)={WeighT(Vec(sum(j=1, #q, gcd(t, q[j])*x^lcm(t, q[j])) + O(x*x^k), -k))}
R(q, n)={vector(n, t, x*Ser(K(q, t, n)/t))}
a(n)={if(n==0, 1, my(s=0); forpart(q=n, my(u=R(q, n)); s+=permcount(q)*polcoef(exp(sum(t=1, n, u[t]-subst(u[t], x, x^2), O(x*x^n))) - exp(sum(t=1, n\2, x^t*u[t] - subst(x^t*u[t], x, x^2), O(x*x^n)))*(1+x), n)); s/n!)} \\ Andrew Howroyd, May 30 2023

Crossrefs

Cf. A007716, A049311, A281116, A283877, A316980, A317752, A317755, A317757, A319616.

Cf. A319077, A319748, A319755, A319778, A319781, A319791.

Sequence in context: A196906 A162057 A026550 * A124124 A052450 A231385

Adjacent sequences: A319748 A319749 A319750 * A319752 A319753 A319754

Keyword

nonn

Author

Gus Wiseman, Sep 27 2018

Extensions

Terms a(11) and beyond from Andrew Howroyd, May 30 2023

Status

approved