19781981 has 2 divisors, whose sum is σ = 19781982. Its totient is φ = 19781980.

The previous prime is 19781969. The next prime is 19781987. The reversal of 19781981 is 18918791.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 15476356 + 4305625 = 3934^2 + 2075^2 .

It is an emirp because it is prime and its reverse (18918791) is a distict prime.

It is a cyclic number.

It is not a de Polignac number, because 19781981 - 2¹⁸ = 19519837 is a prime.

It is a super-2 number, since 2×19781981² = 782653544568722, which contains 22 as substring.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (19781987) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 9890990 + 9890991.

It is an arithmetic number, because the mean of its divisors is an integer number (9890991).

Almost surely, 2^19781981 is an apocalyptic number.

It is an amenable number.

19781981 is a deficient number, since it is larger than the sum of its proper divisors (1).

19781981 is an equidigital number, since it uses as much as digits as its factorization.

19781981 is an odious number, because the sum of its binary digits is odd.

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

The square root of 19781981 is about 4447.6938968414. The cubic root of 19781981 is about 270.4518314810.

The spelling of 19781981 in words is "nineteen million, seven hundred eighty-one thousand, nine hundred eighty-one".