

A002966


Egyptian fractions: number of solutions of 1 = 1/x_1 + ... + 1/x_n where 0 < x_1 <= ... <= x_n.
(Formerly M2981)


48




OFFSET

1,3


COMMENTS

All denominators in the expansion 1 = 1/x_1 + ... + 1/x_n are bounded by A000058(n1), i.e., 0 < x_1 <= ... <= x_n < A000058(n1). Furthermore, for a fixed n, x_i <= (n+1i)*(A000058(i1)1).  Max Alekseyev, Oct 11 2012
From R. J. Mathar, May 06 2010: (Start)
This is the leading edge of the triangle A156869. This is also the row n=1 of an array T(n,m) which gives the number of ways to write 1/n as a sum over m (not necessarily distinct) unit fractions:
1, 1, 3, 14, 147, 3462, 294314, ...
1, 2, 10, 108, 2892, 270332, ...
1, 2, 21, 339, 17253, ...
1, 3, 28, 694, 51323, ...
...
T(.,2) = A018892. T(.,3) = A004194. T(.,4) = A020327, T(.,5) = A020328. T(2,6) is computed by D. S. McNeil, who conjectures that the 2nd row is A003167. (End)
If on the other hand, all x_k must be unique, see A006585.  Robert G. Wilson v, Jul 17 2013


REFERENCES

R. K. Guy, Unsolved Problems in Number Theory, D11.
D. Singmaster, The number of representations of one as a sum of unit fractions, unpublished manuscript, 1972.
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).


LINKS

Table of n, a(n) for n=1..8.
Matthew Brendan Crawford, On the Number of Representations of One as the Sum of Unit Fractions, Master's Thesis, Virginia Polytechnic Institute and State University (2019).
Yuya Dan, Representation of one as the sum of unit fractions, International Mathematical Forum 6:1 (2011), pp. 2530.
Jacques Le Normand, C++ code for a(8) [Broken link]
Jacques Le Normand, C++ code for a(8) [Cached copy]
D. Singmaster, The number of representations of one as a sum of unit fractions, Unpublished M.S., 1972
R. G. Wilson, v, Fax to N. J. A. Sloane, Sep 9, 1994, with copy of Scientific American column by Ian Stewart
Index entries for sequences related to Egyptian fractions


EXAMPLE

For n=3 the 3 solutions are {2,3,6}, {2,4,4}, {3,3,3}.
For n=4 the solutions are: {2,3,7,42}, {2,3,8,24}, {2,3,9,18}, {2,3,10,15}, {2,3,12,12}, {2,4,5,20}, {2,4,6,12}, {2,4,8,8}, {2,5,5,10}, {2,6,6,6}, {3,3,4,12}, {3,3,6,6}, {3,4,4,6}, {4,4,4,4}. [Neven Juric, May 14 2008]


PROG

(PARI) a(n, rem=1, mn=1)=if(n==1, return(numerator(rem)==1)); sum(k=max(1\rem+1, mn), n\rem, a(n1, rem1/k, k)) \\ Charles R Greathouse IV, Jan 04 2015


CROSSREFS

Cf. A002967, A006585, A000058, A348625.
Sequence in context: A126933 A073550 A319361 * A075654 A330603 A261006
Adjacent sequences: A002963 A002964 A002965 * A002967 A002968 A002969


KEYWORD

nonn,nice,hard,more


AUTHOR

N. J. A. Sloane, Robert G. Wilson v


EXTENSIONS

a(7) from Jud McCranie, Nov 15 1999. Confirmed by Marc Paulhus.
a(8) from John Dethridge (jcd(AT)ms.unimelb.edu.au) and Jacques Le Normand (jacqueslen(AT)sympatico.ca), Jan 06 2004


STATUS

approved



