Summations

COSC 3101, PROF. J. ELDER 2 Recall: Insertion Sort

COSC 3101, PROF. J. ELDER 3 ∑ i=1..n i = n = ? Arithmetic Sum

COSC 3101, PROF. J. ELDER n-1+n=S n+n-1+n =S (n+1) + (n+1) + (n+1) (n+1) + (n+1) = 2S n (n+1) = 2S

COSC 3101, PROF. J. ELDER 5 Let’s restate this argument using a geometric representation Algebraic argument n-1+n=S n+ +n =S (n+1) (n+1) + (n+1) = 2S n (n+1) = 2S

COSC 3101, PROF. J. ELDER n = number of blue dots n-1+n=S n+ +n =S (n+1) (n+1) + (n+1) = 2S n (n+1) = 2S

COSC 3101, PROF. J. ELDER n = number of blue dots = number of black dots n n-1+n=S n+ +n =S (n+1) (n+1) + (n+1) = 2S n (n+1) = 2S

COSC 3101, PROF. J. ELDER 8 n+1 n+1 n+1 n+1 n+1 n n n n n n There are n(n+1) dots in the grid (n+1) (n+1) + (n+1) = 2S n (n+1) = 2S = number of blue dots = number of black dots n-1+n=S n+ +n =S

COSC 3101, PROF. J. ELDER 9 n+1 n+1 n+1 n+1 n+1 n n n n n n Note =  (# of terms · last term)) (n+1) (n+1) + (n+1) = 2S n (n+1) = 2S = number of blue dots = number of black dots n-1+n=S n+ +n =S 2 1)(n n S  

COSC 3101, PROF. J. ELDER 10 ∑ i=1..n i = n =  (# of terms · last term) Arithmetic Sum True whenever terms increase slowly

COSC 3101, PROF. J. ELDER 11 ∑ i=0..n 2 i = n = ? Geometric Sum

COSC 3101, PROF. J. ELDER 12 Geometric Sum

COSC 3101, PROF. J. ELDER 13 ∑ i=0..n 2 i = n = 2 · last term - 1 Geometric Sum

COSC 3101, PROF. J. ELDER 14 ∑ i=0..n r i = r 0 + r 1 + r r n = ? Geometric Sum

COSC 3101, PROF. J. ELDER 15 S Srrrr...rr S 1r S r1 r1 23nn1 n1 n1  1rrr r n          Geometric Sum

COSC 3101, PROF. J. ELDER 16 ∑ i=0..n r i r1 r1 n1     Geometric Sum θ(rn)θ(rn) Biggest Term When r>1

COSC 3101, PROF. J. ELDER 17 ∑ i=0..n r i = r 0 + r 1 + r r n =  (biggest term) Geometric Increasing True whenever terms increase quickly

COSC 3101, PROF. J. ELDER 18 ∑ i=0..n r i  Geometric Sum When r<1? 1 r n1   

COSC 3101, PROF. J. ELDER 19 ∑ i=0..n r i  Geometric Sum 1 r n1     θ(1) When r<1 Biggest Term

COSC 3101, PROF. J. ELDER 20 ∑ i=0..n r i = r 0 + r 1 + r r n =  (1) Bounded Tail True whenever terms decrease quickly

COSC 3101, PROF. J. ELDER 21 f(i) = 1 ∑ i=1..n f(i) = n n Sum of Shrinking Function

COSC 3101, PROF. J. ELDER 22 f(i) = 1/2 i  Sum of Shrinking Function

COSC 3101, PROF. J. ELDER 23 f(i) = 1/i ∑ i=1..n f(i) = ? n Sum of Shrinking Function

COSC 3101, PROF. J. ELDER 24 ∑ i=1..n 1 / i = 1 / / / / / 5 + …+ 1 / n = ? Harmonic Sum

COSC 3101, PROF. J. ELDER 26 ∑ i=1..n 1 / i = 1 / / / / / 5 + …+ 1 / n =  (log(n)) Harmonic Sum

COSC 3101, PROF. J. ELDER 27 Approximating Sum by Integrating The area under the curve approximates the sum ∑ i=1..n f(i) ≈ ∫ x=1..n f(x) dx

COSC 3101, PROF. J. ELDER 28 Approximating Sums by Integrating: Harmonic Sum

COSC 3101, PROF. J. ELDER 29 Approximating Sums by Integrating: Arithmetic Sums

COSC 3101, PROF. J. ELDER 30 Approximating Sums by Integrating: Geometric Sum

COSC 3101, PROF. J. ELDER 31 Adding Made Easy We will now classify (most) functions f(i) into four classes: –Geometric Like –Arithmetic Like –Harmonic –Bounded Tail For each class, we will give an easy rule for approximating its sum θ( ∑ i=1..n f(i) )

COSC 3101, PROF. J. ELDER 32 Adding Made Easy f(n) n

COSC 3101, PROF. J. ELDER 33 If the terms f(i) grow sufficiently quickly, then the sum will be dominated by the largest term. f(n)  2 Ω(n)  ∑ i=0..n f(i) = θ(f(n)) Geometric Like: Classic example: ∑ i=0..n 2 i = 2 n+1 -1 ≈ 2 f(n)

COSC 3101, PROF. J. ELDER 34 If the terms f(i) grow sufficiently quickly, then the sum will be dominated by the largest term. f(n)  2 Ω(n)  ∑ i=1..n f(i) = θ(f(n)) Geometric Like: For which functions f(i) is this true? How fast and how slow can it grow?

COSC 3101, PROF. J. ELDER 35 ∑ i=1..n (1000) i ≈ 1.001(1000) n = f(n) Even bigger? f(n)  2 Ω(n)  ∑ i=1..n f(i) = θ(f(n)) Geometric Like:

COSC 3101, PROF. J. ELDER 36 2n2n 2i2i ∑ i=1..n 2 2 ≈ 2 2 = 1f(n) No Upper Extreme: Even bigger! f(n)  2 Ω(n)  ∑ i=1..n f(i) = θ(f(n)) Geometric Like:

COSC 3101, PROF. J. ELDER 37 f(n)  2 Ω(n)  ∑ i=1..n f(i) = θ(f(n)) Geometric Like:

COSC 3101, PROF. J. ELDER 38 f(n)  2 Ω(n)  ∑ i=1..n f(i) = θ(f(n)) Geometric Like:

COSC 3101, PROF. J. ELDER 39 Do All functions in 2 Ω(n) have this property? Maybe not. f(n)  2 Ω(n)  ∑ i=1..n f(i) = θ(f(n)) Geometric Like:

COSC 3101, PROF. J. ELDER 40 Functions that oscillate with exponentially increasing amplitude do not have this property. f(n)  2 Ω(n)  ∑ i=1..n f(i) = θ(f(n)) Geometric Like:

COSC 3101, PROF. J. ELDER 41 Functions expressed with +, -, , , exp, log do not oscillate continually. They are well behaved for sufficiently large n. These do have this property. f(n)  2 Ω(n)  ∑ i=1..n f(i) = θ(f(n)) Geometric Like:

COSC 3101, PROF. J. ELDER 42 Adding Made Easy f(n) n

COSC 3101, PROF. J. ELDER 43 If the terms f(i) are increasing or decreasing relatively slowly, then the sum is roughly the number of terms, n, times the final value. Example 1: ∑ i=1..n 1 = n · 1 f(n) = n θ(1)-1  ∑ i=1..n f(i) = θ(n·f(n)) Arithmetic Like:

COSC 3101, PROF. J. ELDER 44 If the terms f(i) are increasing or decreasing relatively slowly, then the sum is roughly the number of terms, n, times the final value. Example 2: ∑ i=1..n i = n f(n) = n θ(1)-1  ∑ i=1..n f(i) = θ(n·f(n)) Arithmetic Like:

COSC 3101, PROF. J. ELDER 45 Note that the final term is within a constant multiple of the middle term: Thus half the terms are roughly the same and the sum is roughly the number of terms, n, times this value ∑ i=1..n i = n/ n ∑ i=1..n i = θ(n · n) f(n) = n θ(1)-1  ∑ i=1..n f(i) = θ(n·f(n)) Arithmetic Like:

COSC 3101, PROF. J. ELDER 46 Is the statement true for this function? ∑ i=1..n i 2 = n 2 f(n) = n θ(1)-1  ∑ i=1..n f(i) = θ(n·f(n)) Arithmetic Like:

COSC 3101, PROF. J. ELDER 47 Again half the terms are roughly the same. ∑ i=1..n i = (n/2) n 2 1 / 4 n 2 f(n) = n θ(1)-1  ∑ i=1..n f(i) = θ(n·f(n)) Arithmetic Like: ∑ i=1..n i 2 = θ(n · n 2 )

COSC 3101, PROF. J. ELDER 48 area of small square  ∑ i=1..n f(i) ≈ area under curve  area of big square f(n) = n θ(1)-1  ∑ i=1..n f(i) = θ(n·f(n)) Arithmetic Like: = n / 2 · f( n / 2 )  n · f(n)

COSC 3101, PROF. J. ELDER 49 ∑ i=1..n i 2 = n 2 f(n) = n θ(1)-1  ∑ i=1..n f(i) = θ(n·f(n)) Arithmetic Like: f(n) = n 2 f( n / 2 ) = θ(f(n)) The key property is = ?

COSC 3101, PROF. J. ELDER 50 Adding Made Easy Half done f(n) n

COSC 3101, PROF. J. ELDER 51 f(i) = 1 ∑ i=1..n f(i) = θ(n) n Sum of Shrinking Function

COSC 3101, PROF. J. ELDER 52 f(i) = ? ∑ i=1..n f(i) = θ(n 1/2 ) n Sum of Shrinking Function 1/i 1/2

COSC 3101, PROF. J. ELDER 53 f(i) = 1/i ∑ i=1..n f(i) = θ(log n) n Sum of Shrinking Function

COSC 3101, PROF. J. ELDER 54 Does the statement hold for the Harmonic Sum? ∑ i=1..n 1 / i = 1 / / / / / 5 + … + 1 / n ∑ i=1..n f(i) = θ(n · f(n)) = ∑ i=1..n 1 / i =θ(log n) θ(1) = θ(n · 1 / n ) ≠ No the statement does not hold! f(n) = n θ(1)-1  ∑ i=1..n f(i) = θ(n·f(n)) Arithmetic Like:

COSC 3101, PROF. J. ELDER 55 Adding Made Easy not included

COSC 3101, PROF. J. ELDER 56 ∑ i=1..n 1 = n · 1 = n · f(n) Intermediate Case: Lower Extreme: ∑ i=1..n 1 / i = θ(n ) = θ(n · f(n)) Upper Extreme: ∑ i=1..n i 1000 = 1 / 1001 n 1001 = 1 / 1001 n · f(n) f(n) = n θ(1)-1  ∑ i=1..n f(i) = θ(n·f(n)) Arithmetic Like:

COSC 3101, PROF. J. ELDER 57 Adding Made Easy Done

COSC 3101, PROF. J. ELDER 58 Adding Made Easy

COSC 3101, PROF. J. ELDER 59 If the terms f(i) decrease towards zero sufficiently quickly, then the sum will be a constant. The classic example ∑ i=0..n 1 / 2 i = / / / 8 + … < 2. f(n)  n -1-Ω(1)  ∑ i=1..n f(i) = θ(1) Bounded Tail:

COSC 3101, PROF. J. ELDER 60 Upper Extreme: ∑ i=1..n 1 / i = θ(1) No Lower Extreme: 2i2i ∑ i=1..n 2 2 = θ(1). 1 All functions in between. f(n)  n -1-Ω(1)  ∑ i=1..n f(i) = θ(1) Bounded Tail:

COSC 3101, PROF. J. ELDER 61 Adding Made Easy Done

COSC 3101, PROF. J. ELDER 62 Summary Geometric Like: If f(n)  2 Ω(n), then ∑ i=1..n f(i) = θ(f(n)). Arithmetic Like: If f(n) = n θ(1)-1, then ∑ i=1..n f(i) = θ(n · f(n)). Harmonic: If f(n) = 1 / n, then ∑ i=1..n f(i) = θ(logn). Bounded Tail: If f(n)  n -1-Ω(1), then ∑ i=1..n f(i) = θ(1). (For +, -, , , exp, log functions f(n))

End of Lecture 3 March 11, 1009