21819781 has 2 divisors, whose sum is σ = 21819782. Its totient is φ = 21819780.

The previous prime is 21819779. The next prime is 21819799. The reversal of 21819781 is 18791812.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 21538881 + 280900 = 4641^2 + 530^2 .

It is a cyclic number.

It is not a de Polignac number, because 21819781 - 2¹ = 21819779 is a prime.

It is a super-2 number, since 2×21819781² = 952205685775922, which contains 22 as substring.

Together with 21819779, it forms a pair of twin primes.

It is a self number, because there is not a number n which added to its sum of digits gives 21819781.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21810781) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10909890 + 10909891.

It is an arithmetic number, because the mean of its divisors is an integer number (10909891).

Almost surely, 2^21819781 is an apocalyptic number.

It is an amenable number.

21819781 is a deficient number, since it is larger than the sum of its proper divisors (1).

21819781 is an equidigital number, since it uses as much as digits as its factorization.

21819781 is an evil number, because the sum of its binary digits is even.

The product of its digits is 8064, while the sum is 37.

The square root of 21819781 is about 4671.1648440191. The cubic root of 21819781 is about 279.4367119569.

The spelling of 21819781 in words is "twenty-one million, eight hundred nineteen thousand, seven hundred eighty-one".