216720021 has 16 divisors (see below), whose sum is σ = 336472704. Its totient is φ = 121502784.

The previous prime is 216720017. The next prime is 216720037. The reversal of 216720021 is 120027612.

It is not a de Polignac number, because 216720021 - 2² = 216720017 is a prime.

It is a Harshad number since it is a multiple of its sum of digits (21).

It is a self number, because there is not a number n which added to its sum of digits gives 216720021.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (216720001) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 96246 + ... + 98471.

It is an arithmetic number, because the mean of its divisors is an integer number (21029544).

Almost surely, 2^216720021 is an apocalyptic number.

216720021 is a gapful number since it is divisible by the number (21) formed by its first and last digit.

It is an amenable number.

216720021 is a deficient number, since it is larger than the sum of its proper divisors (119752683).

216720021 is a wasteful number, since it uses less digits than its factorization.

216720021 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 194780.

The product of its (nonzero) digits is 336, while the sum is 21.

The square root of 216720021 is about 14721.4136889091. The cubic root of 216720021 is about 600.6659466960.

Adding to 216720021 its reverse (120027612), we get a palindrome (336747633).

The spelling of 216720021 in words is "two hundred sixteen million, seven hundred twenty thousand, twenty-one".