Recurrence and master theorem
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This document discusses recurrences and the master method for solving recurrence relations. It defines a recurrence as an equation that describes a function in terms of its value on smaller functions. The master method provides three cases for solving recurrences of the form T(n) = aT(n/b) + f(n). If f(n) is asymptotically smaller than nlogba, the solution is Θ(nlogba). If f(n) is Θ(nlogba), the solution is Θ(nlogba lgn). If f(n) is asymptotically larger and the regularity condition holds, the solution is Θ(f(n)). It provides examples of applying
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Recurrence and master theorem
- 1. Recurrences • The expression: is a recurrence. – Recurrence: an equation that describes a function in terms of its value on smaller functions 1 2 2 1 )( ncn n T nc nT Dr. AMIT KUMAR @JUET
- 2. Master Method/Theorem • Theorem 4.1 (page 73) – for T(n) = aT(n/b)+f(n), n/b may be n/b or n/b. – where a 1, b>1 are positive integers, f(n) be a non- negative function. 1. If f(n)=O(nlogb a-) for some >0, then T(n)= (nlogb a ). 2. If f(n)= (nlogb a ), then T(n)= (nlogb a lg n). 3. If f(n)=(nlogb a+) for some >0, and if af(n/b) cf(n) for some c<1 and all sufficiently large n, then T(n)= (f(n)). Dr. AMIT KUMAR @JUET
- 3. Implications of Master Theorem • Comparison between f(n) and nlogb a (<,=,>) • Must be asymptotically smaller (or larger) by a polynomial, i.e., n for some >0. • In case 3, the “regularity” must be satisfied, i.e., af(n/b) cf(n) for some c<1 . • There are gaps – between 1 and 2: f(n) is smaller than nlogb a , but not polynomially smaller. – between 2 and 3: f(n) is larger than nlogb a , but not polynomially larger. – in case 3, if the “regularity” fails to hold. Dr. AMIT KUMAR @JUET
- 4. Application of Master Theorem • T(n) = 9T(n/3)+n; – a=9,b=3, f(n) =n – nlogb a = nlog3 9 = (n2) – f(n)=O(nlog3 9-) for =1 – By case 1, T(n) = (n2). • T(n) = T(2n/3)+1 – a=1,b=3/2, f(n) =1 – nlogb a = nlog3/2 1 = (n0) = (1) – By case 2, T(n)= (lg n). Dr. AMIT KUMAR @JUET
- 5. Application of Master Theorem • T(n) = 3T(n/4)+nlg n; – a=3,b=4, f(n) =nlg n – nlogb a = nlog4 3 = (n0.793) – f(n)= (nlog4 3+) for 0.2 – Moreover, for large n, the “regularity” holds for c=3/4. • af(n/b) =3(n/4)lg (n/4) (3/4)nlg n = cf(n) – By case 3, T(n) = (f(n))= (nlg n). Dr. AMIT KUMAR @JUET
- 6. Exception to Master Theorem • T(n) = 2T(n/2)+nlg n; – a=2,b=2, f(n) =nlg n – nlogb a = nlog2 2 = (n) – f(n) is asymptotically larger than nlogb a , but not polynomially larger because – f(n)/nlogb a = lg n, which is asymptotically less than n for any >0. – Therefore,this is a gap between 2 and 3. Dr. AMIT KUMAR @JUET
- 7. Where Are the Gaps nlogba f(n), case 2: within constant distances c1 c2 n f(n), case 1, at least polynomially smaller Gap between case 1 and 2 n Gap between case 3 and 2 f(n), case 3, at least polynomially larger Note: 1. for case 3, the regularity also must hold. 2. if f(n) is lg n smaller, then fall in gap in 1 and 2 3. if f(n) is lg n larger, then fall in gap in 3 and 2 4. if f(n)=(nlogba lgk n), then T(n)=(nlogba lgk+1 n). (as exercise) Dr. AMIT KUMAR @JUET