21009987 has 12 divisors (see below), whose sum is σ = 30994080. Its totient is φ = 13708368.

The previous prime is 21009983. The next prime is 21009997. The reversal of 21009987 is 78990012.

It is a happy number.

It is not a de Polignac number, because 21009987 - 2² = 21009983 is a prime.

It is not an unprimeable number, because it can be changed into a prime (21009983) by changing a digit.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 24412 + ... + 25257.

It is an arithmetic number, because the mean of its divisors is an integer number (2582840).

Almost surely, 2^21009987 is an apocalyptic number.

21009987 is a deficient number, since it is larger than the sum of its proper divisors (9984093).

21009987 is a wasteful number, since it uses less digits than its factorization.

21009987 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 49722 (or 49719 counting only the distinct ones).

The product of its (nonzero) digits is 9072, while the sum is 36.

The square root of 21009987 is about 4583.6652364674. The cubic root of 21009987 is about 275.9361462239.

Adding to 21009987 its reverse (78990012), we get a palindrome (99999999).

The spelling of 21009987 in words is "twenty-one million, nine thousand, nine hundred eighty-seven".