1021991981 has 2 divisors, whose sum is σ = 1021991982. Its totient is φ = 1021991980.

The previous prime is 1021991977. The next prime is 1021992011. The reversal of 1021991981 is 1891991201.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 1015696900 + 6295081 = 31870^2 + 2509^2 .

It is a cyclic number.

It is not a de Polignac number, because 1021991981 - 2² = 1021991977 is a prime.

It is a super-2 number, since 2×1021991981² = 2088935218456608722, which contains 22 as substring.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (1021991081) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (17) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 510995990 + 510995991.

It is an arithmetic number, because the mean of its divisors is an integer number (510995991).

Almost surely, 2^1021991981 is an apocalyptic number.

It is an amenable number.

1021991981 is a deficient number, since it is larger than the sum of its proper divisors (1).

1021991981 is an equidigital number, since it uses as much as digits as its factorization.

1021991981 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 11664, while the sum is 41.

The square root of 1021991981 is about 31968.6093066308. The cubic root of 1021991981 is about 1007.2775688444.

The spelling of 1021991981 in words is "one billion, twenty-one million, nine hundred ninety-one thousand, nine hundred eighty-one".