21221724 has 12 divisors (see below), whose sum is σ = 49517384. Its totient is φ = 7073904.

The previous prime is 21221693. The next prime is 21221731. The reversal of 21221724 is 42712212.

It is a tau number, because it is divible by the number of its divisors (12).

It is a self number, because there is not a number n which added to its sum of digits gives 21221724.

It is a congruent number.

It is an unprimeable number.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 884227 + ... + 884250.

Almost surely, 2^21221724 is an apocalyptic number.

It is an amenable number.

21221724 is an abundant number, since it is smaller than the sum of its proper divisors (28295660).

It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.

21221724 is a wasteful number, since it uses less digits than its factorization.

21221724 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1768484 (or 1768482 counting only the distinct ones).

The product of its digits is 448, while the sum is 21.

The square root of 21221724 is about 4606.7042449022. The cubic root of 21221724 is about 276.8600038927.

Adding to 21221724 its reverse (42712212), we get a palindrome (63933936).

It can be divided in two parts, 21221 and 724, that added together give a triangular number (21945 = T₂₀₉).

The spelling of 21221724 in words is "twenty-one million, two hundred twenty-one thousand, seven hundred twenty-four".