21390646 has 4 divisors (see below), whose sum is σ = 32085972. Its totient is φ = 10695322.

The previous prime is 21390631. The next prime is 21390647. The reversal of 21390646 is 64609312.

It is a semiprime because it is the product of two primes.

It is a Smith number, since the sum of its digits (31) coincides with the sum of the digits of its prime factors. Since it is squarefree, it is also a hoax number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21390647) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 5347660 + ... + 5347663.

It is an arithmetic number, because the mean of its divisors is an integer number (8021493).

Almost surely, 2^21390646 is an apocalyptic number.

21390646 is a deficient number, since it is larger than the sum of its proper divisors (10695326).

21390646 is a wasteful number, since it uses less digits than its factorization.

21390646 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 10695325.

The product of its (nonzero) digits is 7776, while the sum is 31.

The square root of 21390646 is about 4625.0022702697. The cubic root of 21390646 is about 277.5926525557.

Adding to 21390646 its reverse (64609312), we get a palindrome (85999958).

The spelling of 21390646 in words is "twenty-one million, three hundred ninety thousand, six hundred forty-six".