OFFSET
0,3
COMMENTS
A magma with element set X is right involutory if (xy)y = x for x,y in X.
LINKS
Andrew Howroyd, Table of n, a(n) for n = 0..40
PROG
(PARI)
B(c, k)=sum(j=0, c\2, if(k%2, 1, 2^(c-2*j))*k^j*binomial(c, 2*j)*(2*j)!/(2^j*j!))
K(v)=my(S=Set(v)); prod(i=1, #S, my(k=S[i], c=#select(t->t==k, v)); B(c, k))
R(v, m)=concat(vector(#v, i, my(t=v[i], g=gcd(t, m)); vector(g, i, t/g)))
a(n)={my(s=0); forpart(p=n, my(v=Vec(p), S=Set(v)); s+=prod(i=1, #S, my(m=S[i], c=#select(t->t==m, v)); (K(R(v, m))/m)^c/c!)); s}
CROSSREFS
KEYWORD
nonn
AUTHOR
Andrew Howroyd, Apr 17 2023
STATUS
approved