Let there be integers with . The values represent the denominations of different coins, where these denominations have greatest
common divisor of 1. The sums of money that can be represented using the given
coins are then given by
(1)
where the
are nonnegativeintegers
giving the numbers of each coin used. If , it is obviously possibly to represent any quantity of
money .
However, in the general case, only some quantities can be produced. For example, if the allowed coins are , it is impossible to represent
and 3, although all other quantities
can be represented.
Determining the function
giving the greatest
for which there is no solution is called the coin problem, or sometimes the money-changing
problem. The largest such
for a given problem is called the Frobenius number.
The result
(2)
(3)
(Nijenhuis and Wilf 1972) is mathematical folklore. The total number of such nonrepresentable amounts is given by
(4)
The largest nonrepresentable amounts for two coins with denominations and are summarized below.
2
3
1
4
7
17
2
5
3
4
9
23
2
7
5
5
6
19
2
9
7
5
7
23
3
4
5
5
8
27
3
5
7
5
9
31
3
7
11
6
7
29
3
8
13
7
8
41
3
10
17
7
9
47
4
5
11
7
10
53
Fast algorithms are also known for three numbers, but the more general problem for an arbitrary number of numbers is known to be NP-hard
if is fixed (Kannan 1992) or is variable (Ramírez-Alfonsín 1996).
There is no closed-form solution for , although a semi-explicit solution is known which allows
values to be computed quickly (Selmer and Beyer 1978, Rödseth 1978, Greenberg
1988; Beck and Robins 2006). Values for small are summarized below.
Beck, M. and Robins, S. "The Coin-Exchange Problem of Frobenius." §C1 in Computing the Continuous Discretely. New York:
Springer, pp. 3-23, 2006. http://math.sfsu.edu/beck/ccd.html.Berlekamp,
E. R.; Conway, J. H; and Guy, R. K. Winning
Ways for Your Mathematical Plays, Vol. 2: Games in Particular. London:
Academic Press, 1982.Brauer, A. "On a Problem of Partitions."
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B. M. "On a Problem of Partitions. II." Amer. J. Math.76,
343-346, 1954.Davison, J. L. "On the Linear Diophantine Problem
of Frobenius." J. Number Th.48, 353-363, 1994.Greenberg,
H. "Solution to a Linear Diophantine Equation for Nonnegative Integers."
J. Algorithms9, 343-353, 1988.Guy, R. K. "The
Money-Changing Problem." §C7 in Unsolved
Problems in Number Theory, 2nd ed. New York: Springer-Verlag, pp. 113-114,
1994.Kannan, R. "Lattice Translates of a Polytope and the Frobenius
Problem." Combinatorica12, 161-177, 1992.Nabiyev,
V. V. Algorithms: From Theory to Applications. Ankara, Turkey: Seckin,
p. 799, 2007.Nijenhuis, A. "A Minimal-Path Algorithm for the
'Money Changing Problem.' " Amer. Math. Monthly86, 832-835, 1979.Nijenhuis,
A. and Wilf, H. S. "Representations of Integers by Linear Forms in Nonnegative
Integers." J. Number Th.4, 98-106, 1972.Ramírez-Alfonsín,
J. L. "Complexity of the Frobenius Problem." Combinatorica16,
143-147, 1996.Ramírez-Alfonsín, J. L. The
Diophantine Frobenius Problem. Oxford, England: Oxford University Press,
2005.Rødseth, Ø. J. "On a Linear Diophantine
Problem of Frobenius." J. reine angew. Math.301, 171-178, 1978.Rødseth,
Ø. J. "On a Linear Diophantine Problem of Frobenius. II." J.
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Math. Monthly85, 562-565, 1978.
integers 
with 
. The values 
represent the denominations of 
different coins, where these denominations have greatest
common divisor of 1. The sums of money that can be represented using the given
coins are then given by
are nonnegative integers
giving the numbers of each coin used. If 
, it is obviously possibly to represent any quantity of
money 
.
However, in the general case, only some quantities 
can be produced. For example, if the allowed coins are 
, it is impossible to represent
and 3, although all other quantities
can be represented.
giving the greatest 
for which there is no solution is called the coin problem, or sometimes the money-changing
problem. The largest such 
for a given problem is called the Frobenius number 
.
for two coins with denominations 
and 
are summarized below.
is fixed (Kannan 1992) or 
is variable (Ramírez-Alfonsín 1996).
, although a semi-explicit solution is known which allows
values to be computed quickly (Selmer and Beyer 1978, Rödseth 1978, Greenberg
1988; Beck and Robins 2006). Values for small 
are summarized below.
.
Wagon, S. "Greedy
Coins."