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 A199767 Numbers n for which sum_{i=1..k} (1+1/p_i) + product_{i=1..k} (1+1/p_i) is an integer, where p_i are the k prime factors of n (with multiplicity). 6
 21, 45, 432, 740, 1728, 3456, 3888, 5616, 12096, 23760, 46656, 52164, 131328, 152064, 186624, 195656, 233280, 311472, 606528, 618192, 746496, 926208, 933120, 979776, 1273536, 1403136, 2985984, 3221456, 3732480, 5178816, 5412096, 5971968, 9704448, 13651200 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS The numbers of the sequence are the solution of the differential equation  n’=(a-k)n-b, which can also be written as A003415(n)=(a-k)*n-A003958(n), where k is the number of prime factors of n, and a is the integer Sum(i=1..k)(1+1/p_i) +prod(1=1..k) (1+1/p_i). The numbers of the sequence satisfy also sum(i=1..k) (1-1/p_i) - product(i=1..k)(1+1/p_i) = some integer. LINKS J. M. Borwein and E. Wong, A survey of results relating to Giuga’s conjecture on primality, May 8, 1995. R. Mestrovic, On a Congruence Modulo n^3 Involving Two Consecutive Sums of Powers, Journal of Integer Sequences, Vol. 17 (2014), 14.8.4. EXAMPLE 740 has prime factors 2, 2, 5, 37. 1+1/2 +1+1/2 +1+1/5 +1+1/37 =967/185 is the sum over 1+1/p_i. (1+1/2) *(1+1/2) *(1+1/5) *(1+1/37)=513/185 is the product over 1+1/p_i. 967/185 +513/185=8 is an integer. MAPLE isA199767 := proc(n)     p := ifactors(n) ;     add(op(2, d)+op(2, d)/op(1, d), d=p) + mul((1+1/op(1, d))^op(2, d), d=p) ;     type(%, 'integer') ; end proc: for n from 20 do     if isA199767(n) then            printf("%d, \n", n);     end if; end do: # R. J. Mathar, Nov 23 2011 CROSSREFS Cf. A003415, A003958, A007850, A198391. Sequence in context: A063500 A102603 A147222 * A044098 A044479 A139581 Adjacent sequences:  A199764 A199765 A199766 * A199768 A199769 A199770 KEYWORD nonn,easy AUTHOR Paolo P. Lava, Nov 22 2011 STATUS approved

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Last modified June 17 19:10 EDT 2019. Contains 324198 sequences. (Running on oeis4.)