Chapter 1: Q25E (page 49)
Calculate using any method you choose. (Hint: 127 is prime.)
Short Answer
is equal to
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Chapter 1: Q25E (page 49)
Calculate using any method you choose. (Hint: 127 is prime.)
is equal to
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Prove or disprove: If a has an inverse modulo b, then b has an inverse modulo a.
What is the least significant decimal digit of ? (Hint: For distinct primesp,q, and any a is not equal to role="math" localid="1658726105638" , we proved the formula role="math" localid="1658726171933" in Section 1.4.2.)
On page 38, we claimed that since about a fraction of n-bit numbers are prime, on average it is sufficient to draw random n -bit numbers before hitting a prime. We now justify this rigorously. Suppose a particular coin has a probability p of coming up heads. How many times must you toss it, on average, before it comes up heads? (Hint: Method 1: start by showing that the correct expression is . Method 2: if E is the average number of coin tosses, show that ).
Show that if a has a multiplicative inverse modulo N, then this inverse is unique (modulo N).
1.38. To see if a number, say , is divisible by , you just add up the digits of its decimalrepresentation, and see if the result is divisible by role="math" localid="1658402816137" .
( , so it is not divisible by ).
To see if the same number is divisible by , you can do this: subdivide the number into pairs ofdigits, from the right-hand end , add these numbers and see if the sum is divisible by (if it's too big, repeat).
How about ? To see if the number is divisible by , subdivide it into triples from the end add these up, and see if the sum is divisible by .
This is true for any prime other than and . That is, for any prime , there is an integer such that in order to see if divides a decimal number , we break into -tuples of decimal digits (starting from the right-hand end), add up these -tuples, and check if the sum is divisible by .
(a) What is the smallest such for ? For ?
(b) Show that is a divisor of .
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