999981781 has 2 divisors, whose sum is σ = 999981782. Its totient is φ = 999981780.

The previous prime is 999981743. The next prime is 999981791. The reversal of 999981781 is 187189999.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 584188900 + 415792881 = 24170^2 + 20391^2 .

It is a cyclic number.

It is not a de Polignac number, because 999981781 - 2¹³ = 999973589 is a prime.

It is a super-2 number, since 2×999981781² = 1999927124663863922, which contains 22 as substring.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (999981791) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 499990890 + 499990891.

It is an arithmetic number, because the mean of its divisors is an integer number (499990891).

Almost surely, 2^999981781 is an apocalyptic number.

It is an amenable number.

999981781 is a deficient number, since it is larger than the sum of its proper divisors (1).

999981781 is an equidigital number, since it uses as much as digits as its factorization.

999981781 is an evil number, because the sum of its binary digits is even.

The product of its digits is 2939328, while the sum is 61.

The square root of 999981781 is about 31622.4885326883. The cubic root of 999981781 is about 999.9939269631.

The spelling of 999981781 in words is "nine hundred ninety-nine million, nine hundred eighty-one thousand, seven hundred eighty-one".