21720775 has 12 divisors (see below), whose sum is σ = 27591984. Its totient is φ = 16952000.

The previous prime is 21720749. The next prime is 21720779. The reversal of 21720775 is 57702712.

It is not a de Polignac number, because 21720775 - 2¹⁷ = 21589703 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21720779) by changing a digit.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 9571 + ... + 11620.

It is an arithmetic number, because the mean of its divisors is an integer number (2299332).

Almost surely, 2^21720775 is an apocalyptic number.

21720775 is a gapful number since it is divisible by the number (25) formed by its first and last digit.

21720775 is a deficient number, since it is larger than the sum of its proper divisors (5871209).

21720775 is a wasteful number, since it uses less digits than its factorization.

21720775 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 21242 (or 21237 counting only the distinct ones).

The product of its (nonzero) digits is 6860, while the sum is 31.

The square root of 21720775 is about 4660.5552244341. Note that the first 3 decimals coincide. The cubic root of 21720775 is about 279.0134284377.

The spelling of 21720775 in words is "twenty-one million, seven hundred twenty thousand, seven hundred seventy-five".