Asked by Sara
A pair of emirps consists of two prime numbers such that reversing the digits of one number gives the other. How many pairs of two-digit emirps exist such that each number in the pair is greater than 11?
Answers
Answered by
MathMate
The list of prime numbers between 11 and 100 are:
11 13 17 19 23 29 31 37 41 43 47 53 59 61 67 71 73 79 83 89 97
So try them one at a time, say
number reversed
11 11 OK
13 31 OK
17 71 OK
19 <i>91 is composite</i> [pair rejected]
23 32 [rejected]
...
97 79 OK
So you'll get a list in no time.
11 13 17 19 23 29 31 37 41 43 47 53 59 61 67 71 73 79 83 89 97
So try them one at a time, say
number reversed
11 11 OK
13 31 OK
17 71 OK
19 <i>91 is composite</i> [pair rejected]
23 32 [rejected]
...
97 79 OK
So you'll get a list in no time.