21009765 has 32 divisors (see below), whose sum is σ = 38958336. Its totient is φ = 9469440.

The previous prime is 21009761. The next prime is 21009827. The reversal of 21009765 is 56790012.

It is not a de Polignac number, because 21009765 - 2² = 21009761 is a prime.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21009761) by changing a digit.

It is a polite number, since it can be written in 31 ways as a sum of consecutive naturals, for example, 6295 + ... + 9035.

It is an arithmetic number, because the mean of its divisors is an integer number (1217448).

Almost surely, 2^21009765 is an apocalyptic number.

It is an amenable number.

21009765 is a deficient number, since it is larger than the sum of its proper divisors (17948571).

21009765 is a wasteful number, since it uses less digits than its factorization.

21009765 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 2829.

The product of its (nonzero) digits is 3780, while the sum is 30.

The square root of 21009765 is about 4583.6410199753. The cubic root of 21009765 is about 275.9351743362.

Adding to 21009765 its reverse (56790012), we get a palindrome (77799777).

The spelling of 21009765 in words is "twenty-one million, nine thousand, seven hundred sixty-five".