21605971 has 2 divisors, whose sum is σ = 21605972. Its totient is φ = 21605970.

The previous prime is 21605951. The next prime is 21605981. The reversal of 21605971 is 17950612.

It is a strong prime.

It is a cyclic number.

It is not a de Polignac number, because 21605971 - 2⁹ = 21605459 is a prime.

It is a Chen prime.

It is not a weakly prime, because it can be changed into another prime (21605911) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10802985 + 10802986.

It is an arithmetic number, because the mean of its divisors is an integer number (10802986).

Almost surely, 2^21605971 is an apocalyptic number.

21605971 is a deficient number, since it is larger than the sum of its proper divisors (1).

21605971 is an equidigital number, since it uses as much as digits as its factorization.

21605971 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 3780, while the sum is 31.

The square root of 21605971 is about 4648.2223483822. Note that the first 3 decimals coincide. The cubic root of 21605971 is about 278.5209896219.

The spelling of 21605971 in words is "twenty-one million, six hundred five thousand, nine hundred seventy-one".