%I A128648
%S A128648 1,2,4,12,60,5,80,720,7920,55440,55440,6160,6160,18480,425040,5525520,
%T A128648 160240080,160240080,53413360,53413360,480720240,480720240,19709529840,
%U A128648 19709529840,39419059680,197095298400,3350620072800,177582863858400
%N A128648 Denominator of Sum[ (-1)^(k+1)*1/(Prime[k]-1), {k,1,n} ].
%C A128648 A128647(n) = {1,1,3,7,41,3,53,437,5167,34189,36037,3833,3987,11521,...} 
               = Numerator of Sum[ (-1)^(k+1)*1/(Prime[k]-1), {k,1,n} ]. A128646(n) 
               = {1,2,4,12,60,10,80,720,7920,55440,55440,18480,18480,18480,425040,
               ...} = Denominator of Sum[ 1/(Prime[k]-1), {k,1,n} ]. Numbers n such 
               that a(n) equals A128646(n) are listed in A128649(n) = {1,2,3,4,5,
               7,8,9,10,11,14,15,16,17,21,22,23,24,25,26,27,28,29,30,31,32,33,34,
               35,65,66,71,...}.
%H A128648 Eric Weisstein, Link to a section of The World of Mathematics. <a href="http:/
               /mathworld.wolfram.com/PrimeSums.html">Prime Sums</a>.
%F A128648 a(n) = Denominator[ Sum[ (-1)^(k+1)*1/(Prime[k]-1), {k,1,n} ] ].
%t A128648 Table[Denominator[Sum[(-1)^(k+1)*1/(Prime[k]-1),{k,1,n}]],{n,1,36}]
%Y A128648 Cf. A128647 = Numerator of Sum[ (-1)^(k+1)*1/(Prime[k]-1), {k, 1, n} 
               ]. Cf. A128646 = Denominator of Sum[ 1/(Prime[k]-1), {k, 1, n} ]. 
               Cf. A128649, A120271, A119686, A006093, A000040.
%Y A128648 Sequence in context: A020106 A099928 A000568 this_sequence A128646 A155747 
               A058254
%Y A128648 Adjacent sequences: A128645 A128646 A128647 this_sequence A128649 A128650 
               A128651
%K A128648 frac,nonn
%O A128648 1,2
%A A128648 Alexander Adamchuk (alex(AT)kolmogorov.com), Mar 18 2007