数学代写-MAT257
时间:2021-01-16
MAT257 Term Test 2 Rejects
The following questions were a part of a question pool for the 2020-21 MAT257 Term Test 2,
but at the end, they were not included.
1. Let A be a rectangle in Rn and let f , g : A → R, where f is integrable on A and g is equal to
f except on finitely many points. Show from basic definitions that g is also integrable on A
and that

A
f =

A
g.
Tip. “From basic definitions” means “not using any of the theorems that came after the definitions
that are necessary to make the question meaningful”. In our case those definitions are those of lower
and upper sums, integrability, and the integral. Yet words like “measure-0”, whether or not they are
relevant, are forbidden.
2. (a) Show that the boundary of a set of content-0 is also of content-0.
(b) Give an example of a set of measure-0 whose boundary is not of measure-0.
3. Show that if f : A → R is integrable on a rectangle A, and if g : R → R is continuous and
bounded on f (A), then g ◦ f is also integrable on A.
4. Show that if a set A ⊂ Rn is Jordan measurable, then there is a finite collection R of nearly
disjoint rectangles in Rn (meaning, disjoint except perhaps for their boundaries) and a sub-
collection R′ of R, such that
(1)

R∈R′
R ⊂ A ⊂

R∈R
R and

R∈R\R′
v(R) <
1
257
.
5. (a) Show that if a set B is bounded, has measure 0, and its characteristic function χB is
integrable, then

χB = 0.
(b) Give an example of a bounded measure-0 set whose characteristic function is not inte-
grable.
6. Show that if a non-negative continuous function f defined on some rectangle A in Rn has
integral equal to 0, then f is identically equal to 0.
7. Prove Young’s inequality: if f is a continuous strictly increasing function on Rwith f (0) = 0
and if a and b are non-negative numbers, then∫ a
0
f (x)dx +
∫ b
0
f −1(y)dy ≥ ab.
Hint. Draw the graph of f and try to interpret the two integrals and the product ab as areas.
8. Suppose A and B are two Jordan measurable subsets of Rn that have the property that for
every t ∈ R, the (n − 1)-dimensional volume of the slice of A at height t (meaning, of
{x ∈ Rn−1 : (x, t) ∈ A}) is equal to the (n − 1)-dimensional volume of the slice of B at height
t. Prove that v(A) = v(B).
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9. Prove that if f : Rn → Rn is smooth and has an invertible differential at 0, then near 0 it
can be written as a composition g1 ◦ g2 ◦ · · · ◦ gn ◦ T , where T is a “permutation map” that
merely permutes the coordinates of x = (x1 x2 . . . xn) ∈ Rn, and each gi changes the value
of only one coordinate; precisely, gi(x1 x2 . . . xn) = (x1 . . . xi−1 hi x j+1 . . . xn), where the
hi : Rn → R are smooth.
10. Let f be a possibly-unbounded function defined on a possibly-unbounded open subset A of
Rn, and assume that f is integrable on A. Let B be an open subset of A. Prove that f is
integrable also on B.
11. Compute the volume of the ellipsoid {(x, y, z) : 2x2 + 3y2 + 5z2 ≤ 1}. You may use the fact
that the volume of the ball of radius R in R3 is 43piR
3.
12. Let f : R2x,y → R be a function that has continuous second derivatives, and let A be a rectangle
in R2x,y
(a) Use Fubini to show that

R
∂x(∂y f ) =

R
∂y(∂x f ).
(b) Use the above result to show that ∂x(∂y f ) = ∂y(∂x f ).
13. Let Tθ be the “rotation by θ” matrix
(
cos θ − sin θ
sin θ cos θ
)
, and let B be a Jordan-measurable subset
of R2. Prove that v(B) = v(TθB).
14. Let A be a subset of Rn, and letU andV be open covers of A. Show that if {φi} is a partition
of unity for A subordinate toU and {ψ j} is a partition of unity for A subordinate toV, then
{φiψ j} is a partition of unity for A subordinate toW B {U ∩ V : U ∈ U, V ∈ V}.
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