21028661 has 2 divisors, whose sum is σ = 21028662. Its totient is φ = 21028660.

The previous prime is 21028619. The next prime is 21028663. The reversal of 21028661 is 16682012.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 17380561 + 3648100 = 4169^2 + 1910^2 .

It is a cyclic number.

It is not a de Polignac number, because 21028661 - 2⁶ = 21028597 is a prime.

Together with 21028663, it forms a pair of twin primes.

It is a Chen prime.

It is equal to p_1331684 and since 21028661 and 1331684 have the same sum of digits, it is a Honaker prime.

It is a self number, because there is not a number n which added to its sum of digits gives 21028661.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21028663) 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 as a sum of consecutive naturals, namely, 10514330 + 10514331.

It is an arithmetic number, because the mean of its divisors is an integer number (10514331).

Almost surely, 2^21028661 is an apocalyptic number.

It is an amenable number.

21028661 is a deficient number, since it is larger than the sum of its proper divisors (1).

21028661 is an equidigital number, since it uses as much as digits as its factorization.

21028661 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 1152, while the sum is 26.

The square root of 21028661 is about 4585.7018001610. The cubic root of 21028661 is about 276.0178741137.

The spelling of 21028661 in words is "twenty-one million, twenty-eight thousand, six hundred sixty-one".