21762781 has 2 divisors, whose sum is σ = 21762782. Its totient is φ = 21762780.

The previous prime is 21762779. The next prime is 21762817. The reversal of 21762781 is 18726712.

It is a happy number.

Together with next prime (21762817) it forms an Ormiston pair, because they use the same digits, order apart.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 13439556 + 8323225 = 3666^2 + 2885^2 .

It is a cyclic number.

It is not a de Polignac number, because 21762781 - 2¹ = 21762779 is a prime.

It is a super-2 number, since 2×21762781² = 947237273707922, which contains 22 as substring.

Together with 21762779, it forms a pair of twin primes.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21762721) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10881390 + 10881391.

It is an arithmetic number, because the mean of its divisors is an integer number (10881391).

Almost surely, 2^21762781 is an apocalyptic number.

It is an amenable number.

21762781 is a deficient number, since it is larger than the sum of its proper divisors (1).

21762781 is an equidigital number, since it uses as much as digits as its factorization.

21762781 is an evil number, because the sum of its binary digits is even.

The product of its digits is 9408, while the sum is 34.

The square root of 21762781 is about 4665.0595923311. The cubic root of 21762781 is about 279.1931748002.

The spelling of 21762781 in words is "twenty-one million, seven hundred sixty-two thousand, seven hundred eighty-one".