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True or false: the type of a second-order equation may change from one region to another.

  • A.True, but only if the equation is nonlinear
  • B.True, when the coefficients depend on the variables
  • C.False, the type being a fixed property of the equation
  • D.False, the discriminant being constant by construction

Answer: B

If \( A \), \( B \) and \( C \) depend on \( x \) and \( y \), the discriminant does too, and it may change sign. The Tricomi equation \( y z_{xx} + z_{yy} = 0 \) is elliptic for \( y > 0 \) and hyperbolic for \( y < 0 \), a mixed type that arises physically at transonic speeds. Linearity is not what decides it

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