Positive solution for linear Diophantine equations

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Discussion Overview

The discussion centers on finding positive integer solutions for linear Diophantine equations of the form ax + by = c, where a, b, and c are natural numbers. Participants explore the conditions under which positive solutions exist and the methods for determining them.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • Some participants note that if c is a multiple of gcd(a, b), there are infinite integer solutions, but they specifically seek positive integer solutions.
  • One participant provides an example, stating that the equation 7x + 6y = 5 cannot have positive solutions, despite having integer solutions like (5, -6).
  • Another participant questions a specific solution provided, suggesting a correction to (5, -5).
  • It is mentioned that all solutions of the form ax + by = c, assuming a and b are relatively prime, can be expressed as x = x0 + kb, y = y0 - ka for any integer k, and that the ability to choose k to ensure both x and y are positive depends on the values of a, b, x0, and y0.

Areas of Agreement / Disagreement

Participants express differing views on the existence of positive solutions for specific equations, and the discussion remains unresolved regarding the general conditions for finding such solutions.

Contextual Notes

Limitations include the dependence on the specific values of a, b, x0, and y0, as well as the conditions under which positive solutions may or may not exist.

pyfgcr
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The linear Diophantine equations: ax+by=c, a,b,c is natural numbers.
If c is a multiple of gcd(a,b), there is infinite integer solutions, and I know how to find x,y.
However, I wonder how to find positive integer solution x,y only.
 
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pyfgcr said:
The linear Diophantine equations: ax+by=c, a,b,c is natural numbers.
If c is a multiple of gcd(a,b), there is infinite integer solutions, and I know how to find x,y.
However, I wonder how to find positive integer solution x,y only.


They may not exists. For example, the equation \,7x+6y=5\, cannot have positive solutions, but it has

solutions, like \,(5,-6)\,

DonAntonio
 
Shouldn't that be (5,-5)?
 
Mensanator said:
Shouldn't that be (5,-5)?


Yes, you're right of course. Thanks.

DonAntonio
 
All solutions of the Diophantine equation ax+ by= c (assuming a, b, relatively prime) are of the form x= x0+ kb, y= y0- ka for k any integer. If you want both x and y positive, you must be able to choose k so that those are positive. Whether that is possible, of course, depends on a, b, x0, and y0.
 

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