21687641 has 2 divisors, whose sum is σ = 21687642. Its totient is φ = 21687640.

The previous prime is 21687623. The next prime is 21687691. The reversal of 21687641 is 14678612.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 16974400 + 4713241 = 4120^2 + 2171^2 .

It is a cyclic number.

It is not a de Polignac number, because 21687641 - 2¹⁰ = 21686617 is a prime.

It is not a weakly prime, because it can be changed into another prime (21687691) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10843820 + 10843821.

It is an arithmetic number, because the mean of its divisors is an integer number (10843821).

Almost surely, 2^21687641 is an apocalyptic number.

It is an amenable number.

21687641 is a deficient number, since it is larger than the sum of its proper divisors (1).

21687641 is an equidigital number, since it uses as much as digits as its factorization.

21687641 is an evil number, because the sum of its binary digits is even.

The product of its digits is 16128, while the sum is 35.

The square root of 21687641 is about 4656.9991410779. Note that the first 3 decimals coincide. The cubic root of 21687641 is about 278.8714823704.

The spelling of 21687641 in words is "twenty-one million, six hundred eighty-seven thousand, six hundred forty-one".