1,3
a(n) = A000217(A000720(n)-1)*A089819(n);
for n>2: a(n) = A089818(n,2).
a(n) = (pi(n)*(pi(n)-1)*2^(n-pi(n)))/2, with pi = A000720.
a(5)=12 subsets of {1,2,3,4,5} contain exactly two primes: {2,3},
{2,5}, {3,5}, {1,2,3}, {1,2,5}, {1,3,5}, {2,3,4}, {2,4,5}, {3,4,5},
{1,2,3,4}, {1,2,4,5} and {1,3,4,5}.
Cf. A089821.
Sequence in context: A059322 A163908 A108720 * A079352 A089888 A181809
Adjacent sequences: A089819 A089820 A089821 * A089823 A089824 A089825
nonn
Reinhard Zumkeller (reinhard.zumkeller(AT)gmail.com), Nov 12 2003
