A128648 a(n) = denominator(Sum_{k=1..n} (-1)^(k+1)/(prime(k)-1)).

1, 2, 4, 12, 60, 5, 80, 720, 7920, 55440, 55440, 6160, 6160, 18480, 425040, 5525520, 160240080, 160240080, 53413360, 53413360, 480720240, 480720240, 19709529840, 19709529840, 39419059680, 197095298400, 3350620072800, 177582863858400

Offset: 1

Author

Alexander Adamchuk, Mar 18 2007

Keywords

• frac
• nonn

Comments

Numbers m such that a(m) equals A128646(m) are listed in A128649.

Links

• Eric Weisstein's World of Mathematics, Prime Sums

Crossrefs

Cf. A128647 (numerator(Sum_{k=1..n} (-1)^(k+1)/(prime(k)-1))).

Cf. A128646 (denominator(Sum_{k=1..n} 1/(prime(k)-1))).

Cf. A128649, A120271, A119686, A006093, A000040.

Programs

Mathematica

Table[Denominator[Sum[(-1)^(k+1)*1/(Prime[k]-1),{k,1,n}]],{n,1,36}]

Formula

a(n) = denominator(Sum_{k=1..n} (-1)^(k+1)/(prime(k)-1)).