21009691 has 2 divisors, whose sum is σ = 21009692. Its totient is φ = 21009690.

The previous prime is 21009689. The next prime is 21009697. The reversal of 21009691 is 19690012.

It is a weak prime.

It is a cyclic number.

It is not a de Polignac number, because 21009691 - 2¹ = 21009689 is a prime.

Together with 21009689, it forms a pair of twin primes.

It is a Chen prime.

It is not a weakly prime, because it can be changed into another prime (21009697) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10504845 + 10504846.

It is an arithmetic number, because the mean of its divisors is an integer number (10504846).

Almost surely, 2^21009691 is an apocalyptic number.

21009691 is a deficient number, since it is larger than the sum of its proper divisors (1).

21009691 is an equidigital number, since it uses as much as digits as its factorization.

21009691 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 972, while the sum is 28.

The square root of 21009691 is about 4583.6329477828. The cubic root of 21009691 is about 275.9348503722.

The spelling of 21009691 in words is "twenty-one million, nine thousand, six hundred ninety-one".