21686221 has 2 divisors, whose sum is σ = 21686222. Its totient is φ = 21686220.

The previous prime is 21686219. The next prime is 21686227. The reversal of 21686221 is 12268612.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 19236996 + 2449225 = 4386^2 + 1565^2 .

It is a cyclic number.

It is not a de Polignac number, because 21686221 - 2¹ = 21686219 is a prime.

Together with 21686219, it forms a pair of twin primes.

It is a self number, because there is not a number n which added to its sum of digits gives 21686221.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21686227) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10843110 + 10843111.

It is an arithmetic number, because the mean of its divisors is an integer number (10843111).

Almost surely, 2^21686221 is an apocalyptic number.

It is an amenable number.

21686221 is a deficient number, since it is larger than the sum of its proper divisors (1).

21686221 is an equidigital number, since it uses as much as digits as its factorization.

21686221 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 2304, while the sum is 28.

The square root of 21686221 is about 4656.8466798897. The cubic root of 21686221 is about 278.8653958601.

The spelling of 21686221 in words is "twenty-one million, six hundred eighty-six thousand, two hundred twenty-one".