21127781 has 2 divisors, whose sum is σ = 21127782. Its totient is φ = 21127780.

The previous prime is 21127759. The next prime is 21127783. The reversal of 21127781 is 18772112.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 13653025 + 7474756 = 3695^2 + 2734^2 .

It is a cyclic number.

It is not a de Polignac number, because 21127781 - 2¹⁰ = 21126757 is a prime.

It is a super-2 number, since 2×21127781² = 892766259967922, which contains 22 as substring.

Together with 21127783, 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 (21127783) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10563890 + 10563891.

It is an arithmetic number, because the mean of its divisors is an integer number (10563891).

Almost surely, 2^21127781 is an apocalyptic number.

It is an amenable number.

21127781 is a deficient number, since it is larger than the sum of its proper divisors (1).

21127781 is an equidigital number, since it uses as much as digits as its factorization.

21127781 is an evil number, because the sum of its binary digits is even.

The product of its digits is 1568, while the sum is 29.

The square root of 21127781 is about 4596.4966006732. The cubic root of 21127781 is about 276.4508707416.

Adding to 21127781 its reverse (18772112), we get a palindrome (39899893).

The spelling of 21127781 in words is "twenty-one million, one hundred twenty-seven thousand, seven hundred eighty-one".