21820781 has 2 divisors, whose sum is σ = 21820782. Its totient is φ = 21820780.

The previous prime is 21820769. The next prime is 21820793. The reversal of 21820781 is 18702812.

It is a balanced prime because it is at equal distance from previous prime (21820769) and next prime (21820793).

It can be written as a sum of positive squares in only one way, i.e., 21808900 + 11881 = 4670^2 + 109^2 .

It is a cyclic number.

It is not a de Polignac number, because 21820781 - 2¹⁰ = 21819757 is a prime.

It is a super-2 number, since 2×21820781² = 952292966899922, which contains 22 as substring.

It is a Chen prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21820751) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10910390 + 10910391.

It is an arithmetic number, because the mean of its divisors is an integer number (10910391).

Almost surely, 2^21820781 is an apocalyptic number.

It is an amenable number.

21820781 is a deficient number, since it is larger than the sum of its proper divisors (1).

21820781 is an equidigital number, since it uses as much as digits as its factorization.

21820781 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 1792, while the sum is 29.

The square root of 21820781 is about 4671.2718824748. The cubic root of 21820781 is about 279.4409807508.

The spelling of 21820781 in words is "twenty-one million, eight hundred twenty thousand, seven hundred eighty-one".