21870521 has 4 divisors (see below), whose sum is σ = 21901092. Its totient is φ = 21839952.

The previous prime is 21870473. The next prime is 21870547. The reversal of 21870521 is 12507812.

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

It can be written as a sum of positive squares in 2 ways, for example, as 11560000 + 10310521 = 3400^2 + 3211^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21870521 is a prime.

It is a super-2 number, since 2×21870521² = 956639377622882, which contains 22 as substring.

It is an alternating number because its digits alternate between even and odd.

It is a Duffinian number.

It is a Curzon number.

It is not an unprimeable number, because it can be changed into a prime (21870421) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 14186 + ... + 15651.

It is an arithmetic number, because the mean of its divisors is an integer number (5475273).

Almost surely, 2^21870521 is an apocalyptic number.

It is an amenable number.

21870521 is a deficient number, since it is larger than the sum of its proper divisors (30571).

21870521 is an equidigital number, since it uses as much as digits as its factorization.

21870521 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 30570.

The product of its (nonzero) digits is 1120, while the sum is 26.

The square root of 21870521 is about 4676.5928837135. The cubic root of 21870521 is about 279.6531461878.

The spelling of 21870521 in words is "twenty-one million, eight hundred seventy thousand, five hundred twenty-one".