20721218 has 12 divisors (see below), whose sum is σ = 36156582. Its totient is φ = 8880480.

The previous prime is 20721209. The next prime is 20721223. The reversal of 20721218 is 81212702.

It can be written as a sum of positive squares in only one way, i.e., 20638849 + 82369 = 4543^2 + 287^2 .

It is a junction number, because it is equal to n+sod(n) for n = 20721193 and 20721202.

It is an unprimeable number.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 5 ways as a sum of consecutive naturals, for example, 105623 + ... + 105818.

Almost surely, 2^20721218 is an apocalyptic number.

20721218 is a deficient number, since it is larger than the sum of its proper divisors (15435364).

20721218 is a wasteful number, since it uses less digits than its factorization.

20721218 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 211457 (or 211450 counting only the distinct ones).

The product of its (nonzero) digits is 448, while the sum is 23.

The square root of 20721218 is about 4552.0564583493. The cubic root of 20721218 is about 274.6661204182. Note that the first 3 decimals are identical.

The spelling of 20721218 in words is "twenty million, seven hundred twenty-one thousand, two hundred eighteen".