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A085098
a(n) = number of solutions (x_1, x_2, ..., x_n) to Product_{i=1..n} (1 + 1/x_i) = 2.
6
1, 1, 5, 43, 876, 49513, 13005235
OFFSET
1,3
COMMENTS
Number of ways 2 is a product of n superparticular ratios, without regard to order. A superparticular ratio is a ratio of the form m/(m-1). The question relates to music theory, in that various permutations of these products result in scales. - Gene Ward Smith, Apr 11 2006
EXAMPLE
For n = 1, a(1) = 1, one solution: {x_1} = {1}.
For n = 2, a(2) = 1, one solution: {x_1, x_2} = {2, 3}.
For n = 3, a(3) = 5, five solutions: {x_1, x_2, x_3} = {3, 4, 5}, {3, 3, 8}, {2, 4, 15}, {2, 5, 9}, {2, 6, 7}.
In other words, a(3) = 5 since 2 can be written as (4/3)^2 (9/8), (4/3)(5/4)(6/5), (3/2)(7/6)(8/7), (3/2)(6/5)(10/9) or (3/2)(5/4)(16/15) but in no other way using superparticular rations.
MAPLE
spsubdiv := proc(r::rational, n::integer) # Maple program by David Canright local i, j, l, s; # option remember; if n=1 then if numer(r)=denom(r)+1 then [r] else ( NULL ) end if; else s := NULL; for i from floor(1/(r-1))+1 while (1+1/i)^n >= r do l := [spsubdiv( r/(1+1/i), n-1 )]; for j to nops(l) do if op(1, op(j, l)) <= (1+1/i) then s := s, [(1+1/i), op(op(j, l))]; end if od; od; s; end if; end: spl := proc(r, n) [spsubdiv(r, n)] end: spcount := proc(r, n) nops(spl(r, n)) end:
CROSSREFS
Cf. A118086.
Sequence in context: A255895 A160450 A114604 * A271679 A350875 A099794
KEYWORD
hard,more,nonn
AUTHOR
Philippe Deléham, Aug 10 2003
EXTENSIONS
a(5) corrected and a(6) computed by Gene Ward Smith, Apr 11 2006
a(7) was found by Hugo van der Sanden, Dec 14 2007
Edited by N. J. A. Sloane, Oct 18 2008 at the suggestion of Max Alekseyev
STATUS
approved