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Day 31

Sunday 19 April 2020 at 23:27

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Edited by Richard Walker, Sunday 19 April 2020 at 23:29

Here I am at day 31. Can we get anything interesting out of the number 31 though, like we did for 23? Here's a start.

1. It's prime, and if we write the primes up to and including 31 in order

2,3,5,7,11,13,17,19,23,29,31

the median is 13, which is 31 with its digits reversed.

2. If you add all the prime numbers up to and including 31 you get 137, another prime number.

3. If you turn 31 upside down, it is still a number, and it is the same number. How many numbers up to a hundred have this property?

4. If 3. was too easy; what if we went up to a million?