81649621 has 2 divisors, whose sum is σ = 81649622. Its totient is φ = 81649620.

The previous prime is 81649571. The next prime is 81649637. The reversal of 81649621 is 12694618.

It is a happy number.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 71216721 + 10432900 = 8439^2 + 3230^2 .

It is a cyclic number.

It is not a de Polignac number, because 81649621 - 2⁹ = 81649109 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (81649651) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (19) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 40824810 + 40824811.

It is an arithmetic number, because the mean of its divisors is an integer number (40824811).

Almost surely, 2^81649621 is an apocalyptic number.

It is an amenable number.

81649621 is a deficient number, since it is larger than the sum of its proper divisors (1).

81649621 is an equidigital number, since it uses as much as digits as its factorization.

81649621 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 20736, while the sum is 37.

The square root of 81649621 is about 9036.0179836032. The cubic root of 81649621 is about 433.8284785206.

The spelling of 81649621 in words is "eighty-one million, six hundred forty-nine thousand, six hundred twenty-one".