21609987 has 16 divisors (see below), whose sum is σ = 33092352. Its totient is φ = 12287520.

The previous prime is 21609977. The next prime is 21610003. The reversal of 21609987 is 78990612.

It is not a de Polignac number, because 21609987 - 2⁶ = 21609923 is a prime.

It is not an unprimeable number, because it can be changed into a prime (21609937) 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, 1993 + ... + 6869.

It is an arithmetic number, because the mean of its divisors is an integer number (2068272).

Almost surely, 2^21609987 is an apocalyptic number.

21609987 is a deficient number, since it is larger than the sum of its proper divisors (11482365).

21609987 is a wasteful number, since it uses less digits than its factorization.

21609987 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 5098.

The product of its (nonzero) digits is 54432, while the sum is 42.

The square root of 21609987 is about 4648.6543214139. The cubic root of 21609987 is about 278.5382452067.

Subtracting 21609987 from its reverse (78990612), we obtain a square (57380625 = 7575²).

The spelling of 21609987 in words is "twenty-one million, six hundred nine thousand, nine hundred eighty-seven".