19990781 has 2 divisors, whose sum is σ = 19990782. Its totient is φ = 19990780.

The previous prime is 19990777. The next prime is 19990783. The reversal of 19990781 is 18709991.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 19404025 + 586756 = 4405^2 + 766^2 .

It is a cyclic number.

It is not a de Polignac number, because 19990781 - 2² = 19990777 is a prime.

It is a super-2 number, since 2×19990781² = 799262649979922, which contains 22 as substring.

It is a Sophie Germain prime.

Together with 19990783, it forms a pair of twin primes.

It is a Chen 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 (19990783) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 9995390 + 9995391.

It is an arithmetic number, because the mean of its divisors is an integer number (9995391).

Almost surely, 2^19990781 is an apocalyptic number.

It is an amenable number.

19990781 is a deficient number, since it is larger than the sum of its proper divisors (1).

19990781 is an equidigital number, since it uses as much as digits as its factorization.

19990781 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 40824, while the sum is 44.

The square root of 19990781 is about 4471.1051206609. The cubic root of 19990781 is about 271.4000482229.

The spelling of 19990781 in words is "nineteen million, nine hundred ninety thousand, seven hundred eighty-one".