Math 308 Section-Zhou, Review for Exam 2 1. Laplace transform and its inverse: L{f }(s) = ∫∞ 0 e−st f (t)dt, f (t) = L−1 {F (s)}. 2. Basic properties and formulas: L{eat f (t)}(s) = F (s − a), L{tn f (t)} = (−1)n F (n) (s), b s−a n! , L{eat cos(bt)} = , L{tn eat } = , 2 2 2 2 (s − a) + b (s − a) + b (s − a)n+1 L{eat sin(bt)} = { 3. Step-function: uc (t) = L{f (n) (t)} = sn F (s) − sn−1 f (0) − · · · − f (n−1) (0). 0, t < c , L{uc (t)f (t − c)} = e−cs F (s). 1, t ≥ c f1 (t), t < c1 f2 (t), c1 ≤ t < c2 = f1 (t) − (f1 (t) − f2 (t))uc1 (t) − (f2 (t) − f3 (t))uc2 (t). g(t) = f (t), c2 ≤ t 3 { 4. Impulsive function: δ(t − t0 ) = ∞, t = t0 ∫ b , a f (t)δ(t − t0 )dt = f (t0 ) if a ≤ t0 < b. 0, t = ̸ t0 L{δ(t − t0 )} = e−st0 , u′c (t) = δ(t − c). ∫t 5. Convolution: (f ∗ g)(t) = 0 f (t − τ )g(τ )dτ. L{f ∗ g} = F (s)G(s) where F (s) = L{f (t)}, G(s) = L{g(t)}. 6. Solve I.V.P. by LT: ay ′′ + by ′ + cy = g(t), y(0) = y0 , y ′ (0) = y1 . Denote Y (s) = L{y(t)}, G(s) = L{g(t)}. Y (s) = (as + b)y0 + ay1 G(s) + 2 = s-Free response+Forced response, then 2 as + bs + c as + bs + c 7. Partial Fractions: 1 (s−s1 )···(s−sk ) = a1 s−s1 ak , + · · · + s−s k 1 (s−s0 )k = a1 (s−s0 )1 ak + · · · + (s−s k, 0) 8. Consider solving P (x)y ′′ + Q(x)y ′ + R(x)y = 0. Set p(x) = Q(x) P (x) , q(x) = x0 is ordinary if p, q are continuous at x0 . Otherwise x0 is singular. 9. Ordinary x0 : Use y = ∞ ∑ an (x − x0 )n , y ′ = n=0 ∞ ∑ R(x) P (x) an+1 (n + 1)(x − x0 )n , y ′′ = n=0 y(t) = L−1 {Y (s)}. αs+β (s−a)2 +b2 = A(s−a)+Bb . (s−a)2 +b2 ⇒ y ′′ (x) + p(x)y ′ + q(x)y = 0. ∞ ∑ an+2 (n + 2)(n + 1)(x − x0 )n n=0 Equate the coefficients of like power, find the recurrence relation, solve an in terms of a0 and a1 , separate terms in a0 and a1 . The general solution is y(x) = a0 y1 (x) + a1 y2 (x). 10. The lower bound for the radius ρ of convergence of an (x − x0 )n . Find all roots x̄ of P (x) = 0. Find n=0 the lower bound of all |x̄ − x0 |. x2 y ′′ ∞ ∑ αxy ′ 11. Euler equation + + βy = 0. Solve F (r) = + (α − 1)r + β = 0 for r1,2 = Case (a). r1 ̸= r2 real or (1 − α)2 > 4β. y(x) = c1 xr1 + c2 xr2 . Case (b). r1 = r2 = r or (1 − α)2 = 4β. y(x) = xr [c1 + c2 ln(x)]. Case (c). r = µ ± iν or (1 − α)2 < 4β. y(x) = xµ [c1 cos(ν ln(x)) + c2 sin(ν ln(x)]. r2 1−α± √ (1−α)2 −4β . 2 12. Regular singular x0 : lim (x − x0 )p(x) = p0 , lim (x − x0 )2 q(x) = q0 . r2 + (p0 − 1)r + q0 = 0 ⇒ r-values. x→x0 For x0 = 0, set y(x) = xr ∞ ∑ k=0 ak xk = ∞ ∑ x→x0 ak xk+r . Plugging it into the equation, equate the coefficients of k=0 like power and find the recurrence relation. For a specified r-value, solve for ak from the recurrence relation. 1