OFFSET
0,3
LINKS
Wikipedia, Partition of a set
EXAMPLE
a(4) = 7:
{{1,8}, {2,7}, {3,5}, {4,6}},
{{1,8}, {2,7}, {3,4}, {5,6}},
{{1,8}, {2,4}, {3,6}, {5,7}},
{{1,8}, {2,3}, {4,6}, {5,7}},
{{1,8}, {2,4}, {3,5}, {6,7}},
{{1,3}, {2,4}, {5,7}, {6,8}},
{{1,2}, {3,4}, {5,7}, {6,8}}.
MAPLE
b:= proc(s) option remember; `if`(s={}, 1, (j-> add(`if`(i<j and
(isprime(j+i) xor isprime(j-i)), b(s minus {i, j}), 0), i=s))(max(s)))
end:
a:= n-> b({$1..2*n}):
seq(a(n), n=0..15);
MATHEMATICA
b[s_] := b[s] = If[s == {}, 1, With[{j = Max[s]}, Sum[If[i < j && (PrimeQ[j + i] ~Xor~ PrimeQ[j - i]), b[s ~Complement~ {i, j}], 0], {i, s}]]];
a[n_] := b[Range[2n]];
a /@ Range[0, 15] (* Jean-François Alcover, Aug 25 2021, after Alois P. Heinz *)
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Alois P. Heinz, Mar 02 2021
STATUS
approved