81506021 has 4 divisors (see below), whose sum is σ = 81700044. Its totient is φ = 81312000.

The previous prime is 81505999. The next prime is 81506053. The reversal of 81506021 is 12060518.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 12060518 = 2 ⋅6030259.

It can be written as a sum of positive squares in 2 ways, for example, as 43375396 + 38130625 = 6586^2 + 6175^2 .

It is a cyclic number.

It is not a de Polignac number, because 81506021 - 2¹⁴ = 81489637 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (81506921) 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 in 3 ways as a sum of consecutive naturals, for example, 96380 + ... + 97221.

It is an arithmetic number, because the mean of its divisors is an integer number (20425011).

Almost surely, 2^81506021 is an apocalyptic number.

It is an amenable number.

81506021 is a deficient number, since it is larger than the sum of its proper divisors (194023).

81506021 is a wasteful number, since it uses less digits than its factorization.

81506021 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 194022.

The product of its (nonzero) digits is 480, while the sum is 23.

The square root of 81506021 is about 9028.0685088229. The cubic root of 81506021 is about 433.5739995807.

Adding to 81506021 its reverse (12060518), we get a palindrome (93566539).

The spelling of 81506021 in words is "eighty-one million, five hundred six thousand, twenty-one".