• 40 can be written using four 4's:
• A symmetric shape tiled with the 40 welded polysticks of sizes 3, 4, and 5, i.e. those that contain at least a junction point:
• Deleting all the even digits from 240 = 1099511627776 we obtain a prime (199511777).
40 is nontrivially palindromic in base 3, base 7 and base 9.
40 is an esthetic number in base 12, because in such base its adjacent digits differ by 1.
It is a tau number, because it is divible by the number of its divisors (8).
40 is an admirable number.
It is a super Niven number, because it is divisible the sum of any subset of its (nonzero) digits.
40 is an idoneal number.
40 is a nontrivial repdigit in base 3, base 7 and base 9.
It is a plaindrome in base 3, base 7, base 9, base 11, base 12, base 14, base 15 and base 16.
It is a nialpdrome in base 3, base 4, base 7, base 8, base 9, base 10 and base 13.
It is a zygodrome in base 3, base 7 and base 9.
It is a panconsummate number.
A polygon with 40 sides can be constructed with ruler and compass.
40 is the 4-th octagonal number.
It is an amenable number.
It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.
40 is a wasteful number, since it uses less digits than its factorization.
40 is an evil number, because the sum of its binary digits is even.
The square root of 40 is about 6.3245553203. The cubic root of 40 is about 3.4199518934.