Math 334 A1 Homework 3 (Due Nov. 5 5pm)

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1 Math 334 A1 Homework 3 Due Nov. 5 5pm No Advanced or Challenge problems will appear in homeworks. Basic Problems Problem Verify that the given functions are solutions of the differential equation, and determine their Wronskian. Solution. We compute Compute the Wronskian: W = det y + y = 0; 1, cost, sint =0 + 0=0; 2 cost + sint = cost+cost = 0; 3 sint + sint = cost+cost = cost sint 1 cost sint 1 cost sint = det Problem Express 1+i in the form R cosθ + i sinθ = R e iθ. Solution. We need Therefore This gives where k can be any integer. Therefore 1 cost sint 0 sint cost = sin 2 t + cos 2 t= cost sint R cosθ = 1, R sinθ = 1. 6 R 2 = 2 R = 2. 7 cosθ = 1, sinθ = 1 θ = π k π 8 π π 1+i=R cos k π + i sin = k π R e i π +2kπ 4. 9 Problem Find all four roots of 1 1/4. Solution. To find all roots, we need to write 1 into the form R e iθ. Clearly R = 1, cosθ = 1, sinθ = 0 thus Now we have 1=e 2kπi, k is any integer /4 = e 2kπi/4 = e kπ 2 i. 11 It is clear that k and k + 4 gives the same root for any k. Therefore the four roots are given by k = 0, 1, 2, 3. Setting k = 0 we obtain 1; Setting k = 1 we obtain e π 2 i = i; Setting k = 2 we obtain 1; Setting k = 3 we obtain i. So finally the four roots are Problem Determine the radius of convergence of the power series Solution. We have Taking the limit nր, we have Therefore the radius of convergence is 1, i, 1, i n n 2 x+2 n 3 n. 13 a n = 1n n 2 3 n. 14 a n+1 a n = 1 n n n L = lim n 3 n 2 = Problem Determine the Taylor series about x 0 for the given function: ρ = L 1 = yx=lnx, x 0 =

2 2 Math 334 A1 Homework 3 Due Nov. 5 5pm Solution. Recall that the Taylor series is given by yx= yx 0 + y x 0x x 0+ y x 0 x x = y n x 0 2 n! Now y = lnx and x 0 = 1. We compute for n 1 x x 0 n. 19 y n x 0= dn dx nlnx N = x=x0 1n+1 n 1! x n N x=x0=1= 1 n+1 n 1!. 20 Note that yx 0=ln1=0. So the desired Taylor series is lnx= 1 n+1 x 1 n. 21 n Problem Rewrite the given expression as a sum whose generic term involves x n : n n 1 a n x n n=2 Solution. WE need to shift n 2 n. This means the sum now starts from 0, and n becomes n + 2. So the sum becomes n+2n+1 a n+2 x n. 23 Problem Consider y x y y = 0, x 0 = 1, 24 a Find the first four terms in each of two solutions y 1 and y 2 unless the series terminates sooner. b By evaluating the Wronskian Wy 1, y 2x 0, show that y 1 and y 2 form a fundamental set of solutions that is y 1, y 2 are linearly independent. Solution. a Write y = a nx x 0 n = a n x 1 n. 25 Substitute into the equation, we have a nx 1 n [x 1 + 1] a nx 1 n a nx 1 n = First compute the first term: a nx 1 n = n n 1 a nx 1 n n=2 Shifting index, we reach a nx 1 n = n +2n +1 a n+2 x 1 n. 28 Now compute the second term [x 1 + 1] a nx 1 n = x 1 n a nx 1 n 1 n a nx 1 n 1 29 = n a nx 1 n n + 1 a n+1x 1 n. 30 Now the equation becomes n + 2 n + 1 a n+2x 1 n n a nx 1 n n + 1 a n+1 x 1 n a nx 1 n = Note that in the above, three sums start from 0 while one starts from 1. we need to write the n = 0 term separately: 2 a 2 a 1 a 0 + [n + 2n + 1 a n+2 n a n n+1 a n+1 a n] = The recurrence relations are 2 a 2 a 1 a 0 = 0 33 n+2n+1 a n+2 n + 1 a n n + 1 a n+1 = 0 n 1 34

3 3 The second relation can be simplified to Solving them one by one, we have n + 2 a n+2 = a n + a n+1. n 1 35 The general solution is yx=a 0 + a 1 x 1+ n = 0 a 2 = 1 2 a a 1 36 n = 1 a 3 = 1 3 a 1+a 1 2= 6 a a 1 37 n = 2 a 4 = 1 4 a 2+a 1 2 3= 4 3 a 0+a 1 = 1 6 a a 1 38 Collecting all the a 0 s and the a 1 s together we have 1 2 a a 1 x a a 1 x a a 1 x yx = a 0 [ x x x 14 + ] So ] + a 1 [ x x x x y 1x = x x x y 2x = x x x x b The Wronskian at x 0 is det y 11 y y 1 1 y = det = So y 1, y 2 are linearly independent. Problem Find the first five nonzero terms in the solution of the problem y x y y = 0, y0 =2, y 0=1. 44 Solution. Write y = a n x n. 45 Substitute into the equation: 0 = a n x n x a n x n a n x n 46 the recurrence relations are Now the initial conditions give We compute We already have five nonzero terms: = n n 1 a n x n 2 x n a n x n 1 a n x n 47 n=2 = n + 2 n + 1 a n+2 x n n a n x n a n x n 48 = 2 a 2 a 0 + [n + 2n + 1 a n+2 n + 1 a n] x n a 2 a 0 = 0, 50 n+2 a n+2 a n = y0 =2 a 0 = 2; y 0=1 a 1 = a 2 = a 0 = 1; 2 53 a 3 = a 1 3 = 1 3 ; 54 n=2 a 4 = a 2 4 = yx=2+ x + x x x4 + 56

4 4 Math 334 A1 Homework 3 Due Nov. 5 5pm Problem Determine a lower bound for the radius of convergence of series solutions about each given point x 0 for the differential equation 1 + x 3 y + 4 x y + 4 y = 0; x 0 = 0, x 0 = Solution. Write the equation into standard form We see that the singular points are solutions to or equivalently y + 4 x 1 + x 3 y + 4 y = x3 x 3 + 1=0. 59 x 3 = To find all such x, we need to write 1=Re iθ. Clearly R = 1. To determine θ we solve which gives θ = π + 2 k π. the solutions are given by cosθ = 1, sinθ = 0 61 x = e i2k+1 π Notice that k and k + 3 gives the same x. Therefore the three roots are given by setting k =0,1,2. k = 0 x=e i π 1 3 = i; k = 1 x= 1; k = 2 x = i Now we discuss x 0 = 0. The distance from 4 to the three roots are: i = 1 64 The smallest distance is 1. So the radius of convergence is at least 1. x 0 = 2. The distances are 0 1 =1; i = 1; i 3 = i = = 3 ; = 3; i = The smallest distance is 3. So the radius of convergence is 3. Problem Find the first four nonzero terms in each of two power series solutions about the origin for e x y + x y = 0 70 Determine the lower bound of radius of convergence. Solution. We write y = a n x n 71 and expand e x =1 + x+ x2 2 + x3 6 + = x n n!. 72 Substituting into the equation we have x n 0 = a n x n + x a n x n 73 n! = 1 + x + x2 2 + x a2 + 6 a 3 x + 12a 4 x a 5 x a 0 x + a 1 x 2 + a 2 x Note that in the above, we expand everything up to x 3, hoping that the recurrence relations would give us the desired four non-zero terms in both y 1 and y 2. If it turns out that this is not the case, we need to expand to higher order. To make the calculation simpler, we notice that finally the solution is written as y = a 0 y 1 + a 1 y 2. 75

5 5 y 1 is obtained by setting a 0 = 1, a 1 = 0 while y 2 is obtained by setting a 0 = 0, a 1 = 1. Finding y 1. Setting a 0 = 1, a 1 = 0 we have 1 + x+ x2 2 + x a2 + 6 a 3 x + 12a 4 x a 5 x x + a 2 x 3 + = 0 76 Carrying out the multiplication, we have we have 2 a a 2 +6 a 3 +1 x + a a a 4 x 2 + a a 3+12a a 5 + a 2 x 3 + =0. 77 These give 2 a 2 = 0, 78 2 a a = 0, 79 a a a 4 = 0, 80 a a a a 5 + a 2 = a 2 = 0, a 3 = 1 6, a 4= 1 12, a 5 = y 1x=1 1 6 x x x We are lucky that we have exactly four nonzero terms. Finding y 2. Setting a 0 = 1, a 1 = 1 we have 1 + x + x2 2 + x a2 + 6 a 3 x + 12a 4 x a 5 x x 2 + a 2 x 3 + = Carrying out the multiplication, we have 2 a a 2 +6 a 3 x + a a a x 2 + The recurrence relations are which give We only have 3 nonzero terms! a a 3+12a a 5 + a 2 x 3 + = a 2 = 0, 86 2 a a 3 = 0, 87 a a a = 0, 88 a a a a 5 + a 2 = 0, 89 a 2 = 0; a 3 = 0; a 4 = 1 12 ; a 5= y 2 = x 1 12 x x Finding the 4th term. To find the 4th term, we need to expand everything to higher power. Let s try expanding to x 4 : 1+x+ x2 2 + x3 6 + x a2 + 6 a 3 x + 12a 4 x a 5 x a 6 x 4 + x 2 + a 2 x 3 + a 3 x 4 = This gives a new recurrence relation via setting coefficients of x 4 to be 0: a a 3 +6 a a a 6 + a 3 = We obtain The updated y 2 is now Now we have 4 nonzero terms. a 6 = y 2x=x 1 12 x x x To determine the lower bound of the radius of convergence, we need to find all z such that e z = 0, as the standard form of our equation is Write z = α+iβ. We have y + x ex y = e z = e α [cosβ + i sinβ]. 97

6 6 Math 334 A1 Homework 3 Due Nov. 5 5pm e z = e α 0 98 for any real number α. Therefore e z is never zero and the equation does not have any singular point. Consequently the radius of convergence is. Problem Find the general solution Solution. This is Euler equation. Set y = x r we reach So the general solution is Problem Find all singular points of and determine whether each one is regular or irregular. Solution. Write the equation into standard form: We see that there are two singular points x =0, x = 1. At x = 0, we have x 2 y + 4 x y + 2 y = r r 1+4r + 2=0 r 1,2 = 2, y = C 1 x 2 + C 2 x x 2 1 x y + x 2 y 3 x y =0, 102 y + x 2 3 x 2 1 x y y = x1 x x p= x 2 x 1 x, x2 q = 3 x 1 x. 104 We see that x p is not analytic still has singularity at 0. So x = 0 is an irregular singuar point. At x = 1, we have x 1 p = x 2 x 2, x 31 x 12 q = x both are analytic at x = 1. So x = 1 is a regular singular point. Intermediate Problems Problem Determine whether the given set of functions is linearly dependent or linearly independent. If they are linearly dependent, find a linear relation among them. 105 f 1t=2t 3, f 2t=2t 2 + 1, f 3t=3t 2 + t. 106 Note: As f 1, f 2, f 3 are not solutions to some 3rd order equation, Wronskian 0 implies linear independence, but Wronskian = 0 does not imply linear dependence. Finding a linear relation means finding constants C 1, C 2, C 3 such that C 1 f 1 + C 2 f 2 + C 3 f 3 = Solution. We compute the Wronskian if it 0, the functions are linearly independent; If it = 0, we have to use other methods to determine. f 1 f 2 f 3 W = det f 1 f 2 f 3 = det 2 t 3 2 t t 2 + t 2 4 t 6 t+1 = f 1 f 2 f 3 Unfortunately this does not guarantee linear dependence of f 1, f 2, f 3. However, this indicates that we should try to show linear dependence. As an alternative method, we try to directly find C 1, C 2, C 3 such that C 12 t 3 + C 2 2 t C 3 3 t 2 + t = As the left hand side is a polynomial special case of power series the above is equivalent to that coefficients for 1, t, t 2 all vanish. We rewrite the above equation to This gives 3 C 1 + C 2+2 C 1 + C 3 t+2 C C 3 t 2 = C 1 + C 2 = C 1 + C 3 = C C 3 =

7 7 Solving this system, we have C 2 = 3 C 1, C 3 = 2 C 1, C 1 arbitrary 114 So f 1, f 2, f 3 are linearly dependent, a linear relation is given by f f 2 2 f 3 = Problem Find the general solution of y y y + y = Solution. This is linear equation with constant coefficients. The characteristic equation is r 3 r 2 r + 1= Clearly r = 1 is a solution. Write r 3 r 2 r +1 = r 1 r 2 1 = r 1 2 r Therefore r 1,2 = 1; r 3 = So the solution is given by y = C 1 e t + C 2 t e t + C 3 e t. 120 Problem Find the general solution of Solution. The characteristic equation is Notice that if we set R = r 2, we have So the four roots are y 4 5 y + 4 y = r 4 5 r 2 +4 = R 2 5 R + 4=0 R = 4, r 1,2,3,4 = ± 2, ± They are all different, so the general solution is given by yx=c 1 e 2t + C 2 e 2t + C 3 e t + C 4 e t. 125