21870821 has 16 divisors (see below), whose sum is σ = 25691904. Its totient is φ = 18230784.

The previous prime is 21870809. The next prime is 21870829. The reversal of 21870821 is 12807812.

It is a cyclic number.

It is not a de Polignac number, because 21870821 - 2¹⁰ = 21869797 is a prime.

It is a super-2 number, since 2×21870821² = 956665622428082, which contains 22 as substring.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21870829) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 61781 + ... + 62133.

It is an arithmetic number, because the mean of its divisors is an integer number (1605744).

Almost surely, 2^21870821 is an apocalyptic number.

It is an amenable number.

21870821 is a deficient number, since it is larger than the sum of its proper divisors (3821083).

21870821 is a wasteful number, since it uses less digits than its factorization.

21870821 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 580.

The product of its (nonzero) digits is 1792, while the sum is 29.

The square root of 21870821 is about 4676.6249582364. The cubic root of 21870821 is about 279.6544248582.

The spelling of 21870821 in words is "twenty-one million, eight hundred seventy thousand, eight hundred twenty-one".