diff --git a/2bc0c8-2024_spring_TA/Makefile b/2bc0c8-2024_spring_TA/Makefile new file mode 100644 index 0000000..54ed1fb --- /dev/null +++ b/2bc0c8-2024_spring_TA/Makefile @@ -0,0 +1,13 @@ +main: main_ hw16_17 function_analysis_intro lecture_9 + +main_: main.typ + typst compile --font-path ../fonts/ ./main.typ ../build/2bc0c8-2024_spring_TA_main.pdf + +hw16_17: hw16_17.typ + typst compile --font-path ../fonts/ ./hw16_17.typ ../build/2bc0c8-2024_spring_TA_hw16_17.pdf + +function_analysis_intro: function_analysis_intro.typ + typst compile --font-path ../fonts/ ./function_analysis_intro.typ ../build/2bc0c8-2024_spring_TA_function_analysis_intro.pdf + +lecture_9: lecture_9.typ + typst compile --font-path ../fonts/ ./lecture_9.typ ../build/2bc0c8-2024_spring_TA_lecture_9.pdf \ No newline at end of file diff --git a/2bc0c8-2024_spring_TA/hw16_17.typ b/2bc0c8-2024_spring_TA/hw16_17.typ new file mode 100644 index 0000000..d5b351a --- /dev/null +++ b/2bc0c8-2024_spring_TA/hw16_17.typ @@ -0,0 +1,492 @@ +#set text( + font: ("linux libertine", "Source Han Serif SC", "Source Han Serif"), + size: 10pt, +) +#show math.equation: it => text(size: 10pt, math.display(it)) +#let dcases(..args) = { + let dargs = args.pos().map(it => math.display(it)) + math.cases(..dargs) +} + +#show image: it => align(center, it) +#show heading.where(level: 3): it => highlight(it) + +#let Real = "Re" +#let Imaginary = "Im" + +#set page( + header: context [ + #h(1fr) + #if counter(page).get().first() > 1 [ + *Homework 16 & 17 Ans* + ] + ], + footer: context [ + #let headings = query(selector(heading.where(level: 2)).before(here())) + #if headings.len() > 0 { + text(size: 8pt, headings.last().body) + } + #h(1fr) + #counter(page).display( + "1/1", + both: true, + ) + ], +) + +#align(center)[ + = Homework 16 & 17 Ans + 2024 Spring 数学分析 B2 + + PB21000030 马天开 +] + +== 6.12 Ans + +=== 1(3) + +$ + integral_2^(+oo) (dif x) / (x^u ln x) +$ + ++ $u<=1$, then $1/(x^u ln x)>=1/(x ln x)$, we have: + $ + integral_2^(+oo) (dif x) / (x^u ln x) >= integral_2^(+oo) (dif x) / (x ln x) = lim_(t->+oo) ln ln t = +oo + $ + ++ $u>1$, since $ln x$ is strictly increasing, we have: + + $ + integral_2^(+oo) (dif x) / (x^u ln x) <= integral_2^(+oo) (dif x) / (x^u ln 2) < +oo + $ + + Or more generally, since $1/(ln x) -> 0 quad (x->+oo)$, and $integral_2^(+oo) (dif x)/(x^u)<+oo$, with Dirchelet (Page 301, Theorem 13.7), we can show that: + + $ + integral_2^(+oo) (dif x) / (x^u ln x) < +oo + $ + +So the integral converges for $u>1$. + +=== 1(5) + +$ + integral_0^(+oo) (sin^2 x) / (x^alpha (1+x)) dif x +$ + +First split the integral into two parts: + +$ + I = I_1 + I_2 =integral_0^1 (sin^2 x) / (x^alpha (1+x)) dif x + integral_1^(+oo) (sin^2 x) / (x^alpha (1+x)) dif x +$ + +// Note than when $I=integral_0^(+oo)dif x$ converges, a necessary & sufficient condition is that both $I_1=integral_0^1 dif x$ and $I_2=integral_1^(+oo)dif x$ converge. (This differs from splitting into, say $I_1=integral_0^(+oo) f_1 dif x$ and $I_2=integral_0^(+oo) f_2 dif x$, where the in-convergance of $I$ is not guaranteed by the convergance of $I_1$ and $I_2$, or vice versa.) + ++ We start by discussing $I_1$: + + $ + (sin^2 x) / (x^alpha (1+x)) tilde x^2 / x^alpha = x^(2-alpha) + $ + + So integral $I_1 = integral_0^1 (sin^2 x)/(x^alpha (1+x))$ and $integral_0^1 1/(x^(alpha - 2))$ have the same convergance, and the latter converges if $alpha - 2 < 1$, i.e. $alpha < 3$. + ++ The second part $I_2$ requires some techniques. + + + Let's first show the case for $alpha>0$: + + $ + integral_1^(+oo) (sin^2 x) / (x^alpha (1+x)) dif x = 1 / 2 integral_1^(+oo) (1-cos x) / (x^alpha (1+x)) = 1 / 2( + integral_1^(+oo)1 / (2x^alpha (1+x)) dif x-integral_1^(+oo) (cos 2x) / (2x^alpha (1+x)) dif x + ) = 1 / 2 (I_3 + I_4) + $ + + // This is what I meant by in-convergance of $I_1, I_2$ can't deduce the in-convergance of $I$, luckily in this case we can prove both are under $alpha >=0$ + + It's trivial to prove $I_3$ converges with $alpha >0$. For $I_4$, since $integral_1^A cos 2x dif x$ is always bounded, $1/(x^alpha (1+x))$ is strictly decreasing, with Dirchelet, we can show that $I_4$ converges for $alpha >=0$. + + + With $alpha=0$: + + $I_3=integral_1^(+oo)1/(1+x)dif x$ no longer converges, but $I_4$ remains convergent, so $I_2$ diverges for $alpha=0$ + + + Then there's the case for $alpha<0$: + + $ + integral_1^(+oo) (sin^2 x) / (x^alpha (1+x)) dif x >= integral_1^(+oo) (sin^2 x) / (1+x) dif x -> +oo + $ + + Thus $I_2$ diverges for $alpha<0$. + +To sum up, $I$ converges for $00$: + + $ + ln(1+x^2) / x^alpha tilde x^2 / x^alpha = x^(2-alpha) + $ + + So $I_1$ converges if $2-alpha > -1$, i.e. $alpha < 3$. + ++ For $I_2$, the key is to use *logrithm is smaller than any power function when $x->+oo$*, so for $alpha> 1$ + + $ + 0<(ln (1+x^2)) / x^alpha <= x^((alpha-1) / 2) / x^alpha = 1 / x^((alpha+1) / 2) + $ + + We can show $I_2$ is convergent, for $alpha<=1$, use simple inequality: + + $ + ln(1+x^2) / x^alpha >= 1 / x^alpha >0 + $ + + Thus $I_2$ diverges for $alpha<=1$. To sum up, $I$ converges for $11$, but not $alpha=1$. So the integral is *not* uniformly convergent. + +=== 2(5) + +$ + integral_1^(+oo) e^(-alpha x) (cos x) / x^p dif x quad (0<=alpha<+oo, p>0) +$ + +Given that $abs(e^(-alpha x)) <=1 quad forall 11$: + + Given $abs(integral_1^A cos x)<2$ and $1/x^p arrow.br 0$, using Dirchelet, we can show that $integral_1^(+oo) (cos x) / x^p dif x$ converges. + ++ For $00$: + + $ + integral_1^(+oo) (cos x) / (x^p) = integral_1^(pi / 2) (cos x) / (x^p) + sum_(n=1)^oo integral_(pi / 2 + ( + n-1 + ) pi)^(pi / 2 + n pi) (cos x) / (x^p) + $ + + This is a alternating series, after this step, don't use Mean Value Theorem for integrals, which you have no control of $xi_n$ over $[pi/2+(n-1)pi, pi/2+n pi]$. Instead, for each nearby interval, subtract them directly, then one would have: + + $ + integral_(pi / 2+(n-1)pi)^(pi / 2+n pi) (cos x) / (x^p) + (cos (x+pi / 2)) / ((x+pi / 2)^p) >0 + $ + +So the alternating series have a decreasing absolute value, and the integral converges. Thus for $p>0$, the integral uniformly converges. + +=== 2(6) + +$ + integral_0^(+oo) (sin(x^2)) / (1+x^p) dif x quad (0<=p<+oo) +$ + +Same as above, we just prove $integral_0^(+oo) sin(x^2) dif x <+oo$, alongside the fact that $1/(1+x^p) arrow.br quad (p>0)$ (also bounded), this would enough to show uniformly convergent with Abel's Theorem. + +To save some time, we use the same method as above, to turn the integral to a alternating series. However directly invoking the mothod won't work: + +$ + integral_0^(+oo) sin(x^2) = sum_(n=0)^(+oo) integral_(sqrt(n pi))^(sqrt((n+1) pi)) sin(x^2) +$ + +Consider substitute $x^2 -> u$: + +$ + integral_0^(+oo) sin(x^2) dif x &= integral_0^(+oo) (sin x) / (2 sqrt(x)) dif x\ + &= sum_(n=0)^(+oo) integral_(2 n pi)^(2 (n+1) pi) sin(x) / (2 sqrt(x)) dif x +$ + +With the same idea, we subtract the nearby intervals: + +$ + &quad integral_(2 n pi)^(2 (n+1) pi) sin(x) / (2 sqrt(x)) dif x + integral_(2 (n+1) pi)^(2 ( + n+1 + ) pi) sin(x) / (2 sqrt(x)) dif x \ + &=integral_(2 n pi)^(2 (n+1) pi) sin(x) / (2 sqrt(x)) - sin(x) / (2 sqrt(x+pi)) dif x\ + & > 0 +$ + +(skipping some details.) + +Thus the integral converges, and uniformly converges for $p>0$. + +=== 3 + +If $f(x,u)$ is continous on $a<=x<+oo, alpha<=u<=beta$. For every $u in [alpha, beta)$, integral $integral_a^(+oo) f(x,u) dif x$ converges, but not when $u=beta$, which is to say $integral_a^(+oo) f(x, beta) dif x$. Prove that $integral_a^(+oo)f(x,u) dif x$ mustn't be uniformly convergent on $[alpha, beta)$. + +As scary it might seems, this isn't that hard to prove. Using contradiction, let's first say $integral_a^(+oo)f(x,u) dif x$ is uniformly convergent on $[alpha, beta)$, then $forall epsilon > 0, exists X > a, forall A_1,A_2 > X &$ +$ + & abs(integral_(A_1)^(A_2) f(x,u) dif x)beta^-$, we have: + +$ + forall epsilon' > 0, exists delta_u>0, forall u in (beta - delta_u, beta) quad abs(f(x,u) - f(x,beta))=0 + $ + + Since $integral_0^(+oo) 1/(1+x^2)$ is convergent, based on Weierstrass, we know that $F(alpha)$ is unifromly convergent on $[0,+oo)$, thus $F(alpha)$ is continous. + +- Differentiable: + + $ + diff / (diff alpha) ((cos x) / (1+(x+alpha)^2)) = cos x dot (- (2 (x+a)) / ((1+(x+alpha)^2)^2)) + $ + + Control the sizes: + + $ + abs(diff/(diff alpha) ((cos x)/(1+(x+alpha)^2))) <= 2 dot (x+a) / (1+(x+alpha)^2)^2 + $ + + Below is a solution I copied from someone's homework + + $ + integral_0^(+oo) 2 dot (x+a) / (1+(x+alpha)^2)^2 dif x = 2 integral_a^(+oo) x / ( + 1+x^2 + )^2 dif x = integral_sqrt(a)^(+oo) (dif u) / (1+u)^2 1$, so the correct way to do so is just: + + $ + integral_1^(+oo) 2 dot (x+a) / (1+(x+alpha)^2)^2 dif x < 2 dot integral_1^(+oo) x / (1+x^2)^2 dif x + $ + + The range for $alpha$ isn't changed here, only the starting point of the integral, unifromly convergance holds. For proof, recall: uniformly convergent $<=>$ + $ + lim_(A->+oo) sup_(u in [alpha, beta]) abs(integral_A^(+oo) f(x,u) dif x) = 0 + $ + + Or if one could figure it out, use inequality like this: + $ + 2 dot (x+a) / (1+(x+alpha)^2)^2 <= 2 dot (x+a) / (1+(x+alpha)^4) <= 3 / (1+(x+alpha)^3) <= 3 / (1+x^3) + $ + + The following inequality is used here: + $ + 3+3(x+alpha)^4>=2(x+alpha)^4+4(x+alpha)>=2(x+alpha^4)+2(x+alpha)=2(x+alpha)(1+(x+alpha)^3) + $ + +=== 7(2) + +$ + integral_0^(+oo) (1-e^(-a x)) / (x e^x) dif x +$ + + +$ + integral_0^(+oo) (1-e^(-a x)) / (x e^x) dif x &= integral_0^(+oo) (e^(-1 dot x) - e^((-a -1) x)) / (x)dif x\ + &=integral_0^(+oo) dif x integral_(-a-1)^(-1) e^(k x) dif k +$ + +$ + &phi(k) = integral_0^(+oo) e^(k x) dif x = -1 / k quad k <= k_0 < 0 \ +$ + +Consider that: $e^(k x) <= e^(k_0 x)$, also $integral_0^(+oo) e^(k_0 x) <+oo$. $phi(k)$ is uniformly convergent on $[-a-1, -1]$, thus: + +$ + integral_0^(+oo) (1-e^(-a x)) / (x e^x) dif x &= integral_0^(+oo) dif x integral_(-a-1)^(-1) e^(k x) dif k\ + &= integral_(-a-1)^(-1) phi(k) dif k\ + &= [-ln (-k)]_(k=-a-1)^(k=-1) = ln(a+1) +$ + +=== 7(5) + +$ + integral_0^(+oo) (arctan a x) / (x (1+x^2)) dif x +$ + +$ + (arctan a x) / (x (1+x^2)) &= 1 / (x (1+x^2)) arctan x u |_(u = 0)^(u=a) \ + &=integral_0^a 1 / ((1+u^2 x^2)(1+x^2)) dif u +$ + +$ + phi(u) = integral_0^(+oo) 1 / ((1+u^2 x^2)(1+x^2)) dif x +$ + +Consider $1 / ((1+u^2 x^2)(1+x^2)) <= 1/(1+x^2)$. With Weierstrass, $phi(u)$ is uniformly convergent on $[0,a]$, thus: + +$ + integral_0^(+oo) dif x integral_0^a 1 / ((1+u^2 x^2)(1+x^2)) dif u = integral_0^(+a) dif u integral_0^(+oo) 1 / (( + 1+u^2 x^2 + )(1+x^2)) dif x +$ + +$ + integral_0^(+oo) 1 / ((1+u^2 x^2)( + 1+x^2 + )) dif x &= - u^2 / (1-u^2) integral_0^(+oo) 1 / (1+u^2x^2) dif x + 1 / (1-u^2) integral_0^(+oo) 1 / (1+x^2) dif x\ + &= - u / (1-u^2) arctan u x|_0^(+oo) + 1 / (1-u^2) arctan x|_0^(+oo) \ + &= pi / 2(-u / (1-u^2) + 1 / (1-u^2))\ + &= pi / 2 1 / (1+u) +$ + +So the original intergral becomes: + +$ + integral_0^a 1 / (1+u) dif u = pi / 2 ln(a+1) +$ + +=== 7(6) + +$ + integral_0^(+oo) [e^(-(a^2\/x^2))- e^(-(b^2\/ x^2))] dif x +$ + +Wihtout substituting variables, after a few tries, you can see it's impossible to make the form. +$ + &(diff) / (diff u) e^(-u^2\/x^2) = -(2u) / x^2 e^(-u^2\/x^2)\ + &(diff) / (diff u) (1 / u e^(-u^2\/x^2)) = (-(2u) \/ x^2 dot e^(-u^2\/x^2)dot u-e^(-u^2\/x^2)) / (u^2) \ + &(diff) / (diff u) (u e^(-u^2\/x^2)) = e^(-u^2\/x^2) - 2u^2 / x^2 e^(-u^2\/x^2) +$ + +Consider substitute $1\/x -> y$: + +$ + &integral_(oo)^0 [e^(-a^2 y^2)-e^(-b^2 y^2)] (dif y) / (y^2)\ + &=integral_0^(+oo) [e^(-a^2 y^2) - e^(-b^2 y^2)] / (-y^2) dif y\ +$ + +$ + (diff) / (diff v) e^(-v dot y^2) = -y^2 dot e^(-v dot y^2) +$ + +$ + [e^(-a^2 y^2) - e^(-b^2 y^2)] / (-y^2) = integral_b^a e^(-v y^2) dif v +$ + +$ + phi(v) = integral_0^(+oo) e^(-v y^2) dif y = sqrt(pi) / (2 sqrt(v)) +$ + +$ + e^(-v y^2) <= e^(-b y^2) quad forall v in [a,b] +$ + +$ + integral_0^(+oo) [e^(-a^2 y^2) - e^(-b^2 y^2)] / (-y^2) dif y = integral_(a^2)^(b^2) phi(v) dif v = sqrt(pi) dot (b-a) +$ + +=== 8(1) + +$ + &quad integral_(-oo)^(+oo) x / (sigma sqrt(2 pi)) exp(-1/2 ((x-a)/sigma)^2) dif x \ + &=integral_(-oo)^(+oo) (x+a) / (sigma sqrt(2 pi)) exp(-1/2 ((x)/sigma)^2) dif x\ + &=integral_(-oo)^(+oo) x / (sigma sqrt(2 pi)) exp(-1/2 ((x)/sigma)^2) dif x + integral_(-oo)^(+oo) a / (sigma sqrt(2 pi)) exp(-1/2 ((x)/sigma)^2) dif x\ + &= 0 + a / (sigma sqrt(2pi)) integral_(-oo)^(+oo) exp(-1/2 ((x)/sigma)^2) dif x\ + &= a +$ + +This is the expectation of the normal distribution, which is its mean. + +=== 8(3) + + +$ + integral_0^(+oo) (sin a x cos b x) / x dif x &= integral_0^(+oo) (sin (a+b)x + sin(a-b)x) / (2x) dif x\ + &=dcases( + 1/2(pi/2 + pi/2) &= pi/2 quad & a!=b, + 1/2(pi/2 + 0) &= pi/4 quad &a=b, +) +$ + +*Note:* + +$ + integral_0^(+oo) (sin A x) / x dif x = dcases( + integral_0^(+oo) (sin A x)/(A x) dif (A x) = pi/2 quad& A!=0, + 0 quad& A = 0 +) +$ + + +=== 8(6) + +$ + integral_0^(+oo) (sin^4 x) / (x^2) dif x &= integral_0^(+oo) 1 / x^2 (sin^2 x-1 / 4sin^2 2x) dif x\ + &=integral_0^(+oo) ((sin^2 x) / x^2 - (sin^2 2x) / (2x)^2) dif x\ + &=integral_0^(+oo) (sin^2 x) / x^2 dif x - 1 / 2 integral_0^(+oo) (sin^2 2x) / (2x)^2 dif (2x)\ + &=1 / 2integral_0^(+oo) (sin^2 x) / x^2 dif x\ + &=1 / 2integral_o^(+oo) sin^2 x dif (-1 / x)\ + &=1 / 2[-(sin^2 x) / x |_0^(+oo) - integral_0^(+oo)2 / x sin x cos x dif x]\ + &=1 / 2 integral_0^(+oo) (sin 2x) / (2x) dif (2x)\ + &=pi / 4 +$ diff --git a/compile.sh b/compile.sh index 3bfc288..10991cb 100755 --- a/compile.sh +++ b/compile.sh @@ -17,6 +17,12 @@ for folder in $(ls -d */); do if [ "$folder" == "build" ]; then continue fi - echo "Compiling $folder" - typst compile --font-path fonts/ ./$folder/main.typ build/${folder}.pdf + # if folder contains Makefile, run make + if [ -f "$folder/Makefile" ]; then + echo "Running make in $folder" + make -C $folder + else + echo "Compiling $folder" + typst compile --font-path fonts/ ./$folder/main.typ build/${folder}.pdf + fi done \ No newline at end of file