81361981 has 2 divisors, whose sum is σ = 81361982. Its totient is φ = 81361980.

The previous prime is 81361963. The next prime is 81361991. The reversal of 81361981 is 18916318.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 57684025 + 23677956 = 7595^2 + 4866^2 .

It is a cyclic number.

It is not a de Polignac number, because 81361981 - 2¹¹ = 81359933 is a prime.

It is a super-2 number, since 2×81361981² = 13239543904488722, which contains 22 as substring.

It is a self number, because there is not a number n which added to its sum of digits gives 81361981.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (81361991) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 40680990 + 40680991.

It is an arithmetic number, because the mean of its divisors is an integer number (40680991).

Almost surely, 2^81361981 is an apocalyptic number.

It is an amenable number.

81361981 is a deficient number, since it is larger than the sum of its proper divisors (1).

81361981 is an equidigital number, since it uses as much as digits as its factorization.

81361981 is an evil number, because the sum of its binary digits is even.

The product of its digits is 10368, while the sum is 37.

The square root of 81361981 is about 9020.0876381552. The cubic root of 81361981 is about 433.3184404481.

The spelling of 81361981 in words is "eighty-one million, three hundred sixty-one thousand, nine hundred eighty-one".