Upside-Down Triangle, Squared: What the?

Calculus Level pending

Evaluate the Laplacian of the multi-variable function at the point ( π 2 , 3 ) (\frac{\pi}{2}, \; 3)

f ( x , y ) = y e i x + y ln ( i x ) + x 2 f(x, \; y)=ye^{ix}+y \ln (ix) +x^2

Details and Assumptions :

  • i = 1 i=\sqrt{-1}

  • Laplacian of a scalar function : 2 f ( x , y ) = 2 f x 2 + 2 f y 2 \textrm{Laplacian of a scalar function}: \:\: \nabla^2 f(x, \; y)= \frac{\partial^2 f}{\partial x^2} + \frac{\partial^2 f}{\partial y^2}

i π 2 i \frac{\pi}{2} i 3 π 2 i \frac{3 \pi}{2} 1 ( 3 i + 12 π 2 ) 1- \left ( 3i+\frac{12}{\pi^2}\right ) 2 ( 3 i + 12 π 2 ) 2- \left ( 3i+\frac{12}{\pi^2}\right )

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1 solution

Jason Simmons
Dec 16, 2015

First we evaluate the second partial derivative of the function with respect to x x . Then we evaluate the second partial derivative of the function with respect to y y . Executing we find that

2 f x 2 = y e i x y x 2 + 2 \frac{\partial^2 f}{\partial x^2} =-ye^{ix}-\frac{y}{x^2}+2

and

2 f y 2 = 0 \frac{\partial^2 f}{\partial y^2}=0

Once we have found our partial derivatives, we then find the linear combination of the two - per the definition of the Laplacian of a scalar function. Now we have that

2 f ( x , y ) = 2 f x 2 + 2 f y 2 = 2 ( y e i x + y x 2 ) \nabla^2f(x, \; y)= \frac{\partial^2 f}{\partial x^2}+\frac{\partial^2 f}{\partial y^2}= 2- \left (ye^{ix}+\frac{y}{x^2} \right )

and finally, using Euler's Identity e i x = i sin x + cos x e^{ix}=i \sin x +\cos x , we find the answer to the question:

2 f ( π 2 , 3 ) = 2 ( 3 i sin ( π 2 ) + 3 cos ( π 2 ) + 12 π 2 ) = 2 ( 3 i + 12 π 2 ) \nabla^2 f \left (\frac{\pi}{2}, \; 3 \right )=2- \left ( 3i \sin\left ( \frac{\pi}{2} \right ) +3\cos\left ( \frac{\pi}{2} \right )+\frac{12}{\pi^2}\right )=\boxed{2-\left ( 3i+\frac{12}{\pi^2} \right )}

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