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9.2E: Higher Order Constant Coefficient Homogeneous Equations (Exercises)

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Q9.2.1

In Exercises 9.2.1-9.2.14 find the general solution.

1. y3y+3yy=0

2. y(4)+8y9y=0

3. yy+16y16y=0

4. 2y+3y2y3y=0

5. y+5y+9y+5y=0

6. 4y8y+5yy=0

7. 27y+27y+9y+y=0

8. y(4)+y=0

9. y(4)16y=0

10. y(4)+12y+36y=0

11. 16y(4)72y+81y=0

12. 6y(4)+5y+7y+5y+y=0

13. 4y(4)+12y+3y13y6y=0

14. y(4)4y+7y6y+2y=0

Q9.2.2

In Exercises 9.2.15-9.2.27 solve the initial value problem. Graph the solution for Exercises 9.2.17-9.2.19 and 9.2.27.

15. y2y+4y8y=0,y(0)=2,y(0)=2,y(0)=0

16. y+3yy3y=0,y(0)=0,y(0)=14,y(0)=40

17. yyy+y=0,y(0)=2,y(0)=9,y(0)=4

18. y2y4y=0,y(0)=6,y(0)=3,y(0)=22

19. 3yy7y+5y=0,y(0)=145,y(0)=0,y(0)=10

20. y6y+12y8y=0,y(0)=1,y(0)=1,y(0)=4

21. 2y11y+12y+9y=0,y(0)=6,y(0)=3,y(0)=13

22. 8y4y2y+y=0,y(0)=4,y(0)=3,y(0)=1

23. y(4)16y=0,y(0)=2,y(0)=2,y(0)=2,y(0)=0

24. y(4)6y+7y+6y8y=0,y(0)=2,y(0)=8,y(0)=14,y(0)=62

25. 4y(4)13y+9y=0,y(0)=1,y(0)=3,y(0)=1,y(0)=3

26. y(4)+2y2y8y8y=0,y(0)=5,y(0)=2,y(0)=6,y(0)=8

27. 4y(4)+8y+19y+32y+12y=0,y(0)=3,y(0)=3,y(0)=72,y(0)=314

Q9.2.3

28. Find a fundamental set of solutions of the given equation, and verify that it is a fundamental set by evaluating its Wronskian at x=0.

  1. (D1)2(D2)y=0
  2. (D2+4)(D3)y=0
  3. (D2+2D+2)(D1)y=0
  4. D3(D1)y=0
  5. (D21)(D2+1)y=0
  6. (D22D+2)(D2+1)y=0

Q9.2.4

In Exercises 9.2.29-9.2.38 find a fundamental set of solutions.

29. (D2+6D+13)(D2)2D3y=0

30. (D1)2(2D1)3(D2+1)y=0

31. (D2+9)3D2y=0

32. (D2)3(D+1)2Dy=0

33. (D2+1)(D2+9)2(D2)y=0

34. (D416)2y=0

35. (4D2+4D+9)3y=0

36. D3(D2)2(D2+4)2y=0

37. (4D2+1)2(9D2+4)3y=0

38. [(D1)416]y=0

Q9.2.5

39 It can be shown that |111a1a2ana21a22a2nan11an12an1n|=1i<jn(ajai),

where the left side is the Vandermonde determinant and the right side is the product of all factors of the form (ajai) with i and j between 1 and n and i<j.>

  1. Verify (A) for n=2 and n=3.
  2. Find the Wronskian of {ea1x,ea2x,,eanx}.

40. A theorem from algebra says that if P1 and P2 are polynomials with no common factors then there are polynomials Q1 and Q2 such that Q1P1+Q2P2=1. This implies that Q1(D)P1(D)y+Q2(D)P2(D)y=y for every function y with enough derivatives for the left side to be defined.

  1. Use this to show that if P1 and P2 have no common factors and P1(D)y=P2(D)y=0 then y=0.
  2. Suppose P1 and P2 are polynomials with no common factors. Let u1, …, ur be linearly independent solutions of P1(D)y=0 and let v1, …, vs be linearly independent solutions of P2(D)y=0. Use (a) to show that {u1,,ur,v1,,vs} is a linearly independent set.
  3. Suppose the characteristic polynomial of the constant coefficient equation a0y(n)+a1y(n1)++any=0 has the factorization p(r)=a0p1(r)p2(r)pk(r), where each pj is of the form pj(r)=(rrj)nj or pj(r)=[(rλj)2+w2j]mj(ωj>0) and no two of the polynomials p1, p2, …, pk have a common factor. Show that we can find a fundamental set of solutions {y1,y2,,yn} of (A) by finding a fundamental set of solutions of each of the equations pj(D)y=0,1jk, and taking {y1,y2,,yn} to be the set of all functions in these separate fundamental sets.

41.

  1. Show that if z=p(x)cosωx+q(x)sinωx, where p and q are polynomials of degree k, then (D2+ω2)z=p1(x)cosωx+q1(x)sinωx, where p1 and q1 are polynomials of degree k1.
  2. Apply (a) m times to show that if z is of the form (A) where p and q are polynomial of degree m1, then (D2+ω2)mz=0.
  3. Use Equation 9.2.17 to show that if y=eλxz then [(Dλ)2+ω2]my=eλx(D2+ω2)mz.
  4. Conclude from (b) and (c) that if p and q are arbitrary polynomials of degree m1 then y=eλx(p(x)cosωx+q(x)sinωx) is a solution of [(Dλ)2+ω2]my=0.
  5. Conclude from (d) that the functions eλxcosωx,xeλxcosωx,,xm1eλxcosωx,eλxsinωx,xeλxsinωx,,xm1eλxsinωx are all solutions of (C).
  6. Complete the proof of Theorem 9.2.2 by showing that the functions in (D) are linearly independent.

42.

  1. Use the trigonometric identities cos(A+B)=cosAcosBsinAsinBsin(A+B)=cosAsinB+sinAcosB to show that (cosA+isinA)(cosB+isinB)=cos(A+B)+isin(A+B).
  2. Apply (a) repeatedly to show that if n is a positive integer then nk=1(cosAk+isinAk)=cos(A1+A2++An)+isin(A1+A2++An).
  3. Infer from (b) that if n is a positive integer then (cosθ+isinθ)n=cosnθ+isinnθ.
  4. Show that (A) also holds if n=0 or a negative integer. HINT: Verify by direct calculation that (cosθ+isinθ)1=(cosθisinθ). Then replace θ by θ in (A).
  5. Now suppose n is a positive integer. Infer from (A) that if zk=cos(2kπn)+isin(2kπn),k=0,1,,n1, and ζk=cos((2k+1)πn)+isin((2k+1)πn),k=0,1,,n1, then znk=1 and ζnk=1,k=0,1,,n1. (Why don’t we also consider other integer values for k?)
  6. Let ρ be a positive number. Use (e) to show that znρ=(zρ1/nz0)(zρ1/nz1)(zρ1/nzn1) and zn+ρ=(zρ1/nζ0)(zρ1/nζ1)(zρ1/nζn1).

43. Use (e) of Exercise 9.2.42 to find a fundamental set of solutions of the given equation.

  1. yy=0
  2. y+y=0
  3. y(4)+64y=0
  4. y(6)y=0
  5. y(6)+64y=0
  6. [(D1)61]y=0
  7. y(5)+y(4)+y+y+y+y=0

44. An equation of the form a0xny(n)+a1xn1y(n1)++an1xy+any=0,x>0, where a0, a1, …, an are constants, is an Euler or equidimensional equation. Show that if x=et and Y(t)=y(x(t)), then

xdydx=dYdtx2d2ydx2=d2Ydt2dYdtx3d3ydx3=d3Ydt33d2Ydt2+2dYdt.

In general, it can be shown that if r is any integer 2 then

xrdrydxr=drYdtr+A1rdr1Ydtr1++Ar1,rdYdt

where A1r, …, Ar1,r are integers. Use these results to show that the substitution (B) transforms (A) into a constant coefficient equation for Y as a function of t.

45. Use Exercise 9.2.44 to show that a function y=y(x) satisfies the equation a0x3y+a1x2y+a2xy+a3y=0, on (0,) if and only if the function Y(t)=y(et) satisfies a0d3Ydt3+(a13a0)d2Ydt2+(a2a1+2a0)dYdt+a3Y=0. Assuming that a0, a1, a2, a3 are real and a00, find the possible forms for the general solution of (A).


This page titled 9.2E: Higher Order Constant Coefficient Homogeneous Equations (Exercises) is shared under a CC BY-NC-SA 3.0 license and was authored, remixed, and/or curated by William F. Trench via source content that was edited to the style and standards of the LibreTexts platform.

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