21247821 has 24 divisors (see below), whose sum is σ = 35796800. Its totient is φ = 12139848.

The previous prime is 21247819. The next prime is 21247823. The reversal of 21247821 is 12874212.

It is an interprime number because it is at equal distance from previous prime (21247819) and next prime (21247823).

It is not a de Polignac number, because 21247821 - 2¹ = 21247819 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21247823) by changing a digit.

It is a polite number, since it can be written in 23 ways as a sum of consecutive naturals, for example, 355 + ... + 6528.

Almost surely, 2^21247821 is an apocalyptic number.

21247821 is a gapful number since it is divisible by the number (21) formed by its first and last digit.

It is an amenable number.

21247821 is a deficient number, since it is larger than the sum of its proper divisors (14548979).

21247821 is an equidigital number, since it uses as much as digits as its factorization.

21247821 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 6910 (or 6893 counting only the distinct ones).

The product of its digits is 1792, while the sum is 27.

The square root of 21247821 is about 4609.5358768535. The cubic root of 21247821 is about 276.9734451295.

The spelling of 21247821 in words is "twenty-one million, two hundred forty-seven thousand, eight hundred twenty-one".