• 31 can be written using four 4's:
• Deleting all the even digits from 231 = 2147483648 we obtain a prime (173).
• The lines which pass through pairs of vertices of a regular hexagon (black) meet in further 31 points (red):
It is a happy number.
31 is nontrivially palindromic in base 2 and base 5.
31 is an esthetic number in base 7, base 9, base 14 and base 15, because in such bases its adjacent digits differ by 1.
It is a weak prime.
31 is a truncatable prime.
It is a cyclic number.
It is a Chen prime.
It is the 6-th Hogben number.
31 is a lucky number.
31 is a nontrivial repdigit in base 2 and base 5.
It is a plaindrome in base 2, base 4, base 5, base 8, base 9, base 11, base 12, base 13, base 14 and base 16.
It is a nialpdrome in base 2, base 5, base 6, base 7, base 10 and base 15.
It is a zygodrome in base 2 and base 5.
It is a self number, because there is not a number n which added to its sum of digits gives 31.
It is a congruent number.
It is a panconsummate number.
It is a nontrivial repunit in base 2 and in base 5.
31 is an equidigital number, since it uses as much as digits as its factorization.
31 is an odious number, because the sum of its binary digits is odd.
The square root of 31 is about 5.5677643628. The cubic root of 31 is about 3.1413806524.
Adding to 31 its reverse (13), we get a palindrome (44).