The Frobenius number is the largest value for which the Frobenius
equation

(1)

has no solution, where the are positive integers, is an integer, and the solutions are nonnegative integer. As an example, if the values are 4 and 9, then 23 is the largest unsolvable number.
Similarly, the largest number that is not a McNugget
number (a number obtainable by adding multiples of 6, 9, and 20) is 43.

Finding the Frobenius number of a given problem is known as the coin
problem.