21721651 has 12 divisors (see below), whose sum is σ = 25498152. Its totient is φ = 18449088.

The previous prime is 21721621. The next prime is 21721717. The reversal of 21721651 is 15612712.

It is not a de Polignac number, because 21721651 - 2¹⁵ = 21688883 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21721621) 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 in 11 ways as a sum of consecutive naturals, for example, 3576 + ... + 7498.

It is an arithmetic number, because the mean of its divisors is an integer number (2124846).

Almost surely, 2^21721651 is an apocalyptic number.

21721651 is a deficient number, since it is larger than the sum of its proper divisors (3776501).

21721651 is a wasteful number, since it uses less digits than its factorization.

21721651 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 4050 (or 4043 counting only the distinct ones).

The product of its digits is 840, while the sum is 25.

The square root of 21721651 is about 4660.6492037054. The cubic root of 21721651 is about 279.0171792627.

The spelling of 21721651 in words is "twenty-one million, seven hundred twenty-one thousand, six hundred fifty-one".