OFFSET
1,2
COMMENTS
Parker vector for K_3-free graphs.
REFERENCES
P. J. Cameron, Portrait of a typical sum-free set, Surveys in combinatorics 1987, London Math. Soc. Lecture Note Ser., 123, 1987, pp. 13-42.
LINKS
Andrew Howroyd, Table of n, a(n) for n = 1..100
D. A. Gewurz and F. Merola, Sequences realized as Parker vectors of oligomorphic permutation groups, J. Integer Seq., 6 (2003), 03.1.6
EXAMPLE
a(3) = 1, as {} is the only symmetric sum-free set ({1} is not symmetric, while {1,2} is not sum-free). a(4)=3; its symmetric sum-free subsets are {}, {1,3}, {2}.
PROG
(PARI)
a(n)={
my(accept(b, k)=for(i=1, k, if(bittest(b, i), if(bittest(b, min(k+i, n-k-i)) || bittest(b, k-i), return(0)))); 1);
my(recurse(k, b)=if(2*k > n, 1, self()(k+1, b) + if(accept(b + (1<<k), k), self()(k+1, b + (1<<k)))));
recurse(1, 0);
} \\ Andrew Howroyd, Jan 12 2020
CROSSREFS
KEYWORD
nonn
AUTHOR
Daniele A. Gewurz (gewurz(AT)mat.uniroma1.it), Francesca Merola (merola(AT)mat.uniroma1.it), May 06 2003
EXTENSIONS
Terms a(32) and beyond from Andrew Howroyd, Jan 12 2020
STATUS
approved