How this calculation works
Find the largest amount that cannot be made with unlimited supplies of two coprime denominations.
F=ab−a−b for coprime positive denominationsA worked example
With denominations 4 and 7, the largest unattainable total is 17.
Good to know
If the denominations share a factor above one, unattainable totals are unbounded.
Reference: NIST DLMF, Functions of Number Theory (opens in a new tab) ↗
Read the result
Find the largest amount that cannot be made with unlimited supplies of two coprime denominations.
The primary result is labeled “Largest unattainable total”. Keep the input definitions in view when comparing this result with another method.
Try a related calculation
If the denominations share a factor above one, unattainable totals are unbounded.
For a related exercise, try the greatest common divisor.
A few questions, answered
How can I check the default inputs by hand?
With denominations 4 and 7, the largest unattainable total is 17.
Which rule does this calculator apply?
F=ab−a−b for coprime positive denominations