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The two partial differential equations \(f(x, y, z, p, q) = 0\) and \(g(x, y, z, p, q) = 0\) are said to be compatible if :

  • A.every solution of \(f(x, y, z, p, q) = 0\) is also a solution of \(g(x, y, z, p, q) = 0\)
  • B.every solution of \(g(x, y, z, p, q) = 0\) is also a solution of \(f(x, y, z, p, q) = 0\)
  • C.they have common solution
  • D.they do not have common solution

Answer: C

Two first-order equations are compatible if they have common solution.

That is the definition used in the theory of first-order PDEs (Sneddon): \(f(x, y, z, p, q) = 0\) and \(g(x, y, z, p, q) = 0\) are compatible when they have solutions in common, in practice a one-parameter family; it is not required that every solution of one also solve the other. This is the idea behind Charpit's method, which looks for a second equation compatible with the given one.

The first two options demand that one solution set contain the other, which is far stronger and is not the definition. The last option is the opposite of compatibility.

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