81005661 has 12 divisors (see below), whose sum is σ = 127645440. Its totient is φ = 49094280.

The previous prime is 81005657. The next prime is 81005681. The reversal of 81005661 is 16650018.

It is not a de Polignac number, because 81005661 - 2² = 81005657 is a prime.

It is a super-2 number, since 2×81005661² = 13123834228093842, which contains 22 as substring.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (81005641) by changing a digit.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 409021 + ... + 409218.

It is an arithmetic number, because the mean of its divisors is an integer number (10637120).

Almost surely, 2^81005661 is an apocalyptic number.

It is an amenable number.

81005661 is a deficient number, since it is larger than the sum of its proper divisors (46639779).

81005661 is a wasteful number, since it uses less digits than its factorization.

81005661 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 818256 (or 818253 counting only the distinct ones).

The product of its (nonzero) digits is 1440, while the sum is 27.

The square root of 81005661 is about 9000.3144945052. The cubic root of 81005661 is about 432.6849505794.

Adding to 81005661 its reverse (16650018), we get a palindrome (97655679).

The spelling of 81005661 in words is "eighty-one million, five thousand, six hundred sixty-one".