21009961 has 4 divisors (see below), whose sum is σ = 24011392. Its totient is φ = 18008532.

The previous prime is 21009941. The next prime is 21009973. The reversal of 21009961 is 16990012.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is a cyclic number.

It is not a de Polignac number, because 21009961 - 2⁹ = 21009449 is a prime.

It is a super-2 number, since 2×21009961² = 882836922443042, which contains 22 as substring.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21009931) 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, 1500705 + ... + 1500718.

It is an arithmetic number, because the mean of its divisors is an integer number (6002848).

Almost surely, 2^21009961 is an apocalyptic number.

It is an amenable number.

21009961 is a deficient number, since it is larger than the sum of its proper divisors (3001431).

21009961 is an equidigital number, since it uses as much as digits as its factorization.

21009961 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 3001430.

The product of its (nonzero) digits is 972, while the sum is 28.

The square root of 21009961 is about 4583.6624003083. The cubic root of 21009961 is about 275.9360323996.

Adding to 21009961 its reverse (16990012), we get a palindrome (37999973).

The spelling of 21009961 in words is "twenty-one million, nine thousand, nine hundred sixty-one".