- #1
kingwinner
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Q1: Suppose B=[0,1]x[0,2]x[0,3]x[0,4] in R4, and that C=[0,1]x[0,1]x[0,1]x[0,1]. Given that
∫ ∫ ∫ ∫f(x)=d4x=(2pi)4
B
What is the value of
∫ ∫ ∫ ∫ f(x1,2x2,3x3,4x4) d4x?
C
Solution:
Define x=G(u)=(u1,u2/2,u3/3,u4/4)
∫ ∫ ∫ ∫ f(x1,2x2,3x3,4x4) d4x
C
by change of variables theorem,
=(1/24) ∫ ∫ ∫ ∫ f(u1,u2,u3,u4) d4u
------B=G-1(C)
=(1/24)∫ ∫ ∫ ∫ f(u)d4u
---------B
=(2pi)4/24
============================================
I don't understand the last step...
We are only given that
∫ ∫ ∫ ∫f(x)=d4x=(2pi)4
B
NOT that
∫ ∫ ∫ ∫f(u)=d4u=(2pi)4
B
I will tell you what I am thinking about. Here, u=(u1,u2,u3,u4) and x=(x1,x2,x3,x4) CANNOT be treated as dummy variables since they have a relationship x=G(u) used to define the transformation G, but the last step of the solution seems to treat that x=u, which makes me feel very uncomfortable...
Can someone explain? I would really appreciate it!
∫ ∫ ∫ ∫f(x)=d4x=(2pi)4
B
What is the value of
∫ ∫ ∫ ∫ f(x1,2x2,3x3,4x4) d4x?
C
Solution:
Define x=G(u)=(u1,u2/2,u3/3,u4/4)
∫ ∫ ∫ ∫ f(x1,2x2,3x3,4x4) d4x
C
by change of variables theorem,
=(1/24) ∫ ∫ ∫ ∫ f(u1,u2,u3,u4) d4u
------B=G-1(C)
=(1/24)∫ ∫ ∫ ∫ f(u)d4u
---------B
=(2pi)4/24
============================================
I don't understand the last step...
We are only given that
∫ ∫ ∫ ∫f(x)=d4x=(2pi)4
B
NOT that
∫ ∫ ∫ ∫f(u)=d4u=(2pi)4
B
I will tell you what I am thinking about. Here, u=(u1,u2,u3,u4) and x=(x1,x2,x3,x4) CANNOT be treated as dummy variables since they have a relationship x=G(u) used to define the transformation G, but the last step of the solution seems to treat that x=u, which makes me feel very uncomfortable...
Can someone explain? I would really appreciate it!
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