21157985 has 8 divisors (see below), whose sum is σ = 25641576. Its totient is φ = 16758400.

The previous prime is 21157973. The next prime is 21157987. The reversal of 21157985 is 58975112.

It can be written as a sum of positive squares in 4 ways, for example, as 9096256 + 12061729 = 3016^2 + 3473^2 .

It is a sphenic number, since it is the product of 3 distinct primes.

It is not a de Polignac number, because 21157985 - 2⁶ = 21157921 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21157987) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 20444 + ... + 21453.

It is an arithmetic number, because the mean of its divisors is an integer number (3205197).

Almost surely, 2^21157985 is an apocalyptic number.

It is an amenable number.

21157985 is a deficient number, since it is larger than the sum of its proper divisors (4483591).

21157985 is a wasteful number, since it uses less digits than its factorization.

21157985 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 42003.

The product of its digits is 25200, while the sum is 38.

The square root of 21157985 is about 4599.7809730464. The cubic root of 21157985 is about 276.5825448674.

The spelling of 21157985 in words is "twenty-one million, one hundred fifty-seven thousand, nine hundred eighty-five".