Required Math - Summations

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Required Math - Summations. The summation symbol should be one of the easiest math symbols for you computer science majors to understand – if you don’t now, you will!. Required Math - Summations.

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Required Math - Summations
• The summation symbol should be one of the easiest math symbols for you computer science majors to understand – if you don’t now, you will!

CS 4953 The Hidden Art of Steganography

Required Math - Summations
• The summation symbol should be one of the easiest math symbols for you computer science majors to understand – if you don’t now, you will!
• This symbol simply means to add up a series of numbers
• The equivalent in C is:
• int j, sum = 0;
• for(j = 0; j < n; j++)
• { sum = sum + Xj; }
• How many times will this loop?

CS 4953 The Hidden Art of Steganography

Required Math - Summations
• n = total number of elements
• P = probability of occurrence of a particular element
• X = element
• j = index
• If pj is the same for all elements (this is called equiprobable), then what is a simple, everyday name for ‘a’?
• Sometimes, ‘n’ is not shown so it just means to SUM over the entire set, whatever the size

CS 4953 The Hidden Art of Steganography

Required Math - Logarithms
• Definition: logbN = x is such that bX = N
• Read as “the logarithm base b of N equals x”
• You have two logarithm buttons on your calculator
• “log” and “ln”
• log is base 10 and ln is base e (e ~ 2.71)
• ln is called the natural log and has numerous scientific applications
• You also have three inverse log buttons
• 10X, eX, yx
• Unfortunately, we will deal with log base 2, and you do not (likely) have this buttons available 
• However, there is a formula to help you out 

CS 4953 The Hidden Art of Steganography

Required Math - Logarithms
• logbN = logaN/logab
• The value of ‘a’ is arbitrary, so you can use either log base 10 or log base ‘e’
• Some other properties of logarithms (base is irrelevant)
• log (N * M) = log N + log M
• note: 10x * 10y = 10(x + y)
• log (N / M) = log N – log M
• log NM = M log N
• logbb = 1 (base matches number)
• log 1 = 0
• log 0 = undefined
• For log base 2, we will use the notation “lg”
• log2N = lg N
• Unless otherwise indicated, log N will imply a base of 10

CS 4953 The Hidden Art of Steganography

Required Math - Logarithms
• Examples – should be able to answer by inspection
• 3 + 5 = ?
• You see how quickly you can answer the above question? That’s how fast you need to be able to answer these:
• log 1?
• log 10?
• log 100?
• log 1000?
• log 10356?
• lg 1?
• lg 2?
• lg 256?
• lg 1024?
• lg 21024?
• Don’t wait, learn the properties now!

CS 4953 The Hidden Art of Steganography

Required Math - Logarithms
• A few other properties:
• Log N > 0 if N > 1
• Log N = 0 if N = 1
• Log N < 0 if N > 0 && N < 1
• What about the inverse lg?
• Example, what is the inverse lg of 8?
• inv log 6?
• inv log 0?
• inv lg 12?

CS 4953 The Hidden Art of Steganography

Background - Probability
• Probability may be defined in several ways
• It may be the likelihood of outcomes of a certain set of events, for example, the flipping of a coin
• How many possible outcomes are there?
• Another way of looking at probability is that it is a measure of human surprise at the outcome
• More surprise, more information, lower probability
• Probability is always between zero and one
• Probability = zero means it will NOT happen (is this EVER true?)
• Probability = one means it will happen (is this ever true?)

CS 4953 The Hidden Art of Steganography