21218861 has 2 divisors, whose sum is σ = 21218862. Its totient is φ = 21218860.

The previous prime is 21218831. The next prime is 21218863. The reversal of 21218861 is 16881212.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 20529961 + 688900 = 4531^2 + 830^2 .

It is a cyclic number.

It is not a de Polignac number, because 21218861 - 2¹⁰ = 21217837 is a prime.

Together with 21218863, it forms a pair of twin primes.

It is a Chen prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21218863) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10609430 + 10609431.

It is an arithmetic number, because the mean of its divisors is an integer number (10609431).

Almost surely, 2^21218861 is an apocalyptic number.

It is an amenable number.

21218861 is a deficient number, since it is larger than the sum of its proper divisors (1).

21218861 is an equidigital number, since it uses as much as digits as its factorization.

21218861 is an evil number, because the sum of its binary digits is even.

The product of its digits is 1536, while the sum is 29.

The square root of 21218861 is about 4606.3934916592. The cubic root of 21218861 is about 276.8475530375.

The spelling of 21218861 in words is "twenty-one million, two hundred eighteen thousand, eight hundred sixty-one".