21688987 has 2 divisors, whose sum is σ = 21688988. Its totient is φ = 21688986.

The previous prime is 21688981. The next prime is 21689021. The reversal of 21688987 is 78988612.

It is a weak prime.

It is a cyclic number.

It is not a de Polignac number, because 21688987 - 2¹¹ = 21686939 is a prime.

It is a self number, because there is not a number n which added to its sum of digits gives 21688987.

It is not a weakly prime, because it can be changed into another prime (21688981) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10844493 + 10844494.

It is an arithmetic number, because the mean of its divisors is an integer number (10844494).

Almost surely, 2^21688987 is an apocalyptic number.

21688987 is a deficient number, since it is larger than the sum of its proper divisors (1).

21688987 is an equidigital number, since it uses as much as digits as its factorization.

21688987 is an evil number, because the sum of its binary digits is even.

The product of its digits is 387072, while the sum is 49.

The square root of 21688987 is about 4657.1436524977. The cubic root of 21688987 is about 278.8772514510.

The spelling of 21688987 in words is "twenty-one million, six hundred eighty-eight thousand, nine hundred eighty-seven".