21009947 has 4 divisors (see below), whose sum is σ = 24011376. Its totient is φ = 18008520.

The previous prime is 21009941. The next prime is 21009973. The reversal of 21009947 is 74990012.

It is a semiprime because it is the product of two primes.

It is a cyclic number.

It is not a de Polignac number, because 21009947 - 2⁴ = 21009931 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21009941) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 1500704 + ... + 1500717.

It is an arithmetic number, because the mean of its divisors is an integer number (6002844).

Almost surely, 2^21009947 is an apocalyptic number.

21009947 is a deficient number, since it is larger than the sum of its proper divisors (3001429).

21009947 is an equidigital number, since it uses as much as digits as its factorization.

21009947 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 3001428.

The product of its (nonzero) digits is 4536, while the sum is 32.

The square root of 21009947 is about 4583.6608731450. The cubic root of 21009947 is about 275.9359711095.

Adding to 21009947 its reverse (74990012), we get a palindrome (95999959).

The spelling of 21009947 in words is "twenty-one million, nine thousand, nine hundred forty-seven".