19781891 has 2 divisors, whose sum is σ = 19781892. Its totient is φ = 19781890.

The previous prime is 19781851. The next prime is 19781953. The reversal of 19781891 is 19818791.

It is a weak prime.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-19781891 is a prime.

It is a super-3 number, since 3×19781891³ (a number of 23 digits) contains 333 as substring.

It is a Chen prime.

It is not a weakly prime, because it can be changed into another prime (19781821) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 9890945 + 9890946.

It is an arithmetic number, because the mean of its divisors is an integer number (9890946).

Almost surely, 2^19781891 is an apocalyptic number.

19781891 is a deficient number, since it is larger than the sum of its proper divisors (1).

19781891 is an equidigital number, since it uses as much as digits as its factorization.

19781891 is an evil number, because the sum of its binary digits is even.

The product of its digits is 36288, while the sum is 44.

The square root of 19781891 is about 4447.6837792271. The cubic root of 19781891 is about 270.4514213316.

The spelling of 19781891 in words is "nineteen million, seven hundred eighty-one thousand, eight hundred ninety-one".