21957781 has 2 divisors, whose sum is σ = 21957782. Its totient is φ = 21957780.

The previous prime is 21957779. The next prime is 21957799. The reversal of 21957781 is 18775912.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 16240900 + 5716881 = 4030^2 + 2391^2 .

It is a cyclic number.

It is not a de Polignac number, because 21957781 - 2¹ = 21957779 is a prime.

It is a super-2 number, since 2×21957781² = 964288292887922, which contains 22 as substring.

Together with 21957779, 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 (21957721) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10978890 + 10978891.

It is an arithmetic number, because the mean of its divisors is an integer number (10978891).

Almost surely, 2^21957781 is an apocalyptic number.

It is an amenable number.

21957781 is a deficient number, since it is larger than the sum of its proper divisors (1).

21957781 is an equidigital number, since it uses as much as digits as its factorization.

21957781 is an evil number, because the sum of its binary digits is even.

The product of its digits is 35280, while the sum is 40.

The square root of 21957781 is about 4685.9130380322. The cubic root of 21957781 is about 280.0245769243.

The spelling of 21957781 in words is "twenty-one million, nine hundred fifty-seven thousand, seven hundred eighty-one".