21687691 has 2 divisors, whose sum is σ = 21687692. Its totient is φ = 21687690.

The previous prime is 21687641. The next prime is 21687697. The reversal of 21687691 is 19678612.

It is a strong prime.

It is a cyclic number.

It is not a de Polignac number, because 21687691 - 2¹⁹ = 21163403 is a prime.

It is a super-2 number, since 2×21687691² = 940711881822962, which contains 22 as substring.

It is not a weakly prime, because it can be changed into another prime (21687697) by changing a digit.

It is a good prime.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10843845 + 10843846.

It is an arithmetic number, because the mean of its divisors is an integer number (10843846).

Almost surely, 2^21687691 is an apocalyptic number.

21687691 is a deficient number, since it is larger than the sum of its proper divisors (1).

21687691 is an equidigital number, since it uses as much as digits as its factorization.

21687691 is an evil number, because the sum of its binary digits is even.

The product of its digits is 36288, while the sum is 40.

The square root of 21687691 is about 4657.0045093386. The cubic root of 21687691 is about 278.8716966793.

The spelling of 21687691 in words is "twenty-one million, six hundred eighty-seven thousand, six hundred ninety-one".