Number theory - gcd and linear diophantine equations

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SUMMARY

The discussion focuses on finding necessary and sufficient conditions for the linear Diophantine equation ax + by = c to have positive integer solutions when gcd(a, b) = 1. It establishes that if either x0 or y0 is negative, one must find another solution (x1, y1) such that both x1 and y1 are greater than zero. The transformation of the original equation leads to the conditions x1 - x0 = kb and y1 - y0 = -ka, where k is an integer, thus reducing the problem to finding appropriate values of k that satisfy x0 + kb > 0 and y0 - ka > 0.

PREREQUISITES
  • Understanding of linear Diophantine equations
  • Knowledge of the greatest common divisor (gcd)
  • Familiarity with integer solutions and their properties
  • Basic algebraic manipulation skills
NEXT STEPS
  • Study the properties of linear Diophantine equations in depth
  • Learn about the implications of gcd in number theory
  • Explore integer solution techniques for equations of the form ax + by = c
  • Investigate the role of parameter k in shifting solutions in Diophantine equations
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Mathematicians, students studying number theory, and anyone interested in solving linear Diophantine equations and understanding their conditions for positive integer solutions.

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Homework Statement



Suppose that gcd(a, b) = 1 with a, b > 0, and let x0, y0 be any integer solution to the equation ax + by = c. Find a necessary and sufficient condition, possibly depending on a, b, c, x0, y0 that the equation have a solution with x > 0 and y > 0.

Homework Equations





The Attempt at a Solution


I'm pretty lost. Can anyone point me in the right direction?
 
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Well, if one of [tex]x_0[/tex] or [tex]y_0[/tex] is negative, then you have to look for another solution [tex](x_1, y_1)[/tex] with [tex]x_1, y_1 > 0[/tex]. By subtracting equations, you get [tex]a(x_1 - x_0) + b(y_1 - y_0) = 0[/tex]. Can you see why this equation implies that [tex]x_1 - x_0 = kb[/tex] and [tex]y_1 - y_0 = -ka[/tex] for some integer [tex]k[/tex]? Once you do, your problem is reduced to a search for a value of [tex]k[/tex] such that [tex]x_0 + kb > 0[/tex] and [tex]y_0 - ka > 0[/tex]. You should be able to take it from here.
 

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