# Complex Variables. Chapter 2. Analytic Functions Section Harmonic Functions Proofs of Theorems. March 19, 2017

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1 Complex Variables Chapter 2. Analytic Functions Section Harmonic Functions Proofs of Theorems March 19, 2017 () Complex Variables March 19, / 5

3 Theorem Theorem Theorem If a function f (z) = u(x, y) + iv(x, y) is analytic in a domain D, then its component functions u(x, y) and v(x, y) are harmonic in D. Proof. In Corollary 4.52.A, we will see that if f (z) = u(x, y) + iv(x, y) is analytic at a point then u(x, y) and v(x, y) have continuous partial derivatives of all orders at the point. Since f is analytic in D, then the Cauchy-Riemann equations are satisfied by Theorem 2.21.A so u x = v y and u y = v x. () Complex Variables March 19, / 5

4 Theorem Theorem Theorem If a function f (z) = u(x, y) + iv(x, y) is analytic in a domain D, then its component functions u(x, y) and v(x, y) are harmonic in D. Proof. In Corollary 4.52.A, we will see that if f (z) = u(x, y) + iv(x, y) is analytic at a point then u(x, y) and v(x, y) have continuous partial derivatives of all orders at the point. Since f is analytic in D, then the Cauchy-Riemann equations are satisfied by Theorem 2.21.A so u x = v y and u y = v x. Differentiating the Cauchy-Riemann equations with respect to x gives u xx = v yx and u yx = v xx. (3) Differentiating the Cauchy-Riemann equations with respect to y gives u xy = v yy and u yy = v xy. (4) () Complex Variables March 19, / 5

5 Theorem Theorem Theorem If a function f (z) = u(x, y) + iv(x, y) is analytic in a domain D, then its component functions u(x, y) and v(x, y) are harmonic in D. Proof. In Corollary 4.52.A, we will see that if f (z) = u(x, y) + iv(x, y) is analytic at a point then u(x, y) and v(x, y) have continuous partial derivatives of all orders at the point. Since f is analytic in D, then the Cauchy-Riemann equations are satisfied by Theorem 2.21.A so u x = v y and u y = v x. Differentiating the Cauchy-Riemann equations with respect to x gives u xx = v yx and u yx = v xx. (3) Differentiating the Cauchy-Riemann equations with respect to y gives u xy = v yy and u yy = v xy. (4) () Complex Variables March 19, / 5

6 Theorem Theorem (continued) Proof (continued). u xx = v yx and u yx = v xx. (3) u xy = v yy and u yy = v xy. (4) By The Mixed Derivative Theorem (Clairaut s Theorem) (see Theorem 2 of my Calculus 3 [MATH 2110] notes: gardnerr/2110/notes-12e/c14s3.pdf) if the first partials and the mixed second partials are continuous then the mixed second partials are equal. So u xy = u yx and v yx = v xy. () Complex Variables March 19, / 5

7 Theorem Theorem (continued) Proof (continued). u xx = v yx and u yx = v xx. (3) u xy = v yy and u yy = v xy. (4) By The Mixed Derivative Theorem (Clairaut s Theorem) (see Theorem 2 of my Calculus 3 [MATH 2110] notes: gardnerr/2110/notes-12e/c14s3.pdf) if the first partials and the mixed second partials are continuous then the mixed second partials are equal. So u xy = u yx and v yx = v xy. From (3) and (4) we have throughout D u xx + u yy = 0 and v xx + v yy = 0. So u(x, y) and v(x, y) are harmonic in D. () Complex Variables March 19, / 5

8 Theorem Theorem (continued) Proof (continued). u xx = v yx and u yx = v xx. (3) u xy = v yy and u yy = v xy. (4) By The Mixed Derivative Theorem (Clairaut s Theorem) (see Theorem 2 of my Calculus 3 [MATH 2110] notes: gardnerr/2110/notes-12e/c14s3.pdf) if the first partials and the mixed second partials are continuous then the mixed second partials are equal. So u xy = u yx and v yx = v xy. From (3) and (4) we have throughout D u xx + u yy = 0 and v xx + v yy = 0. So u(x, y) and v(x, y) are harmonic in D. () Complex Variables March 19, / 5

9 Theorem Theorem Theorem A function f (z) = f (x + iy) = u(x, y) + iv(x, y) is analytic in a domain D if and only if v(x, y) is a harmonic conjugate of u(x, y). Proof. If v is a harmonic conjugate of u, then their first order partial derivatives satisfy the Cauchy-Riemann equations (by definition of harmonic conjugates) throughout D. So by Theorem 2.22.A, f is differentiable throughout D and so f is analytic on D. () Complex Variables March 19, / 5

10 Theorem Theorem Theorem A function f (z) = f (x + iy) = u(x, y) + iv(x, y) is analytic in a domain D if and only if v(x, y) is a harmonic conjugate of u(x, y). Proof. If v is a harmonic conjugate of u, then their first order partial derivatives satisfy the Cauchy-Riemann equations (by definition of harmonic conjugates) throughout D. So by Theorem 2.22.A, f is differentiable throughout D and so f is analytic on D. If f is analytic in D, then by Theorem u and v are harmonic in D. By the definition of analytic, f is differentiable throughout D and so by Theorem 2.21.A, u and v satisfy the Cauchy-Riemann equations on D. So (by the definition of harmonic conjugates), v is a harmonic conjugate of v. () Complex Variables March 19, / 5

11 Theorem Theorem Theorem A function f (z) = f (x + iy) = u(x, y) + iv(x, y) is analytic in a domain D if and only if v(x, y) is a harmonic conjugate of u(x, y). Proof. If v is a harmonic conjugate of u, then their first order partial derivatives satisfy the Cauchy-Riemann equations (by definition of harmonic conjugates) throughout D. So by Theorem 2.22.A, f is differentiable throughout D and so f is analytic on D. If f is analytic in D, then by Theorem u and v are harmonic in D. By the definition of analytic, f is differentiable throughout D and so by Theorem 2.21.A, u and v satisfy the Cauchy-Riemann equations on D. So (by the definition of harmonic conjugates), v is a harmonic conjugate of v. () Complex Variables March 19, / 5

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