21720646 has 16 divisors (see below), whose sum is σ = 33804288. Its totient is φ = 10456200.

The previous prime is 21720631. The next prime is 21720649. The reversal of 21720646 is 64602712.

It is a plaindrome in base 13.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21720649) 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 7 ways as a sum of consecutive naturals, for example, 13041 + ... + 14611.

It is an arithmetic number, because the mean of its divisors is an integer number (2112768).

Almost surely, 2^21720646 is an apocalyptic number.

21720646 is a deficient number, since it is larger than the sum of its proper divisors (12083642).

21720646 is a wasteful number, since it uses less digits than its factorization.

21720646 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1827.

The product of its (nonzero) digits is 4032, while the sum is 28.

The square root of 21720646 is about 4660.5413848608. The cubic root of 21720646 is about 279.0128760817.

It can be divided in two parts, 21720 and 646, that added together give a triangular number (22366 = T₂₁₁).

The spelling of 21720646 in words is "twenty-one million, seven hundred twenty thousand, six hundred forty-six".