21491857781 has 2 divisors, whose sum is σ = 21491857782. Its totient is φ = 21491857780.

The previous prime is 21491857729. The next prime is 21491857817. The reversal of 21491857781 is 18775819412.

Together with next prime (21491857817) it forms an Ormiston pair, because they use the same digits, order apart.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 19793676100 + 1698181681 = 140690^2 + 41209^2 .

It is a cyclic number.

It is not a de Polignac number, because 21491857781 - 2³⁰ = 20418115957 is a prime.

It is a super-2 number, since 2×21491857781² (a number of 21 digits) contains 22 as substring.

It is a Sophie Germain prime.

It is a Curzon number.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21491857081) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10745928890 + 10745928891.

It is an arithmetic number, because the mean of its divisors is an integer number (10745928891).

Almost surely, 2^21491857781 is an apocalyptic number.

It is an amenable number.

21491857781 is a deficient number, since it is larger than the sum of its proper divisors (1).

21491857781 is an equidigital number, since it uses as much as digits as its factorization.

21491857781 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 1128960, while the sum is 53.

The spelling of 21491857781 in words is "twenty-one billion, four hundred ninety-one million, eight hundred fifty-seven thousand, seven hundred eighty-one".