1981561 has 4 divisors (see below), whose sum is σ = 1984500. Its totient is φ = 1978624.

The previous prime is 1981547. The next prime is 1981583. The reversal of 1981561 is 1651891.

It is a semiprime because it is the product of two primes, and also a brilliant number, because the two primes have the same length.

It can be written as a sum of positive squares in 2 ways, for example, as 1428025 + 553536 = 1195^2 + 744^2 .

It is a cyclic number.

It is not a de Polignac number, because 1981561 - 2¹³ = 1973369 is a prime.

It is a super-3 number, since 3×1981561³ = 23342297133379384443, which contains 333 as substring.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (1981361) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 105 + ... + 1993.

It is an arithmetic number, because the mean of its divisors is an integer number (496125).

2^1981561 is an apocalyptic number.

It is an amenable number.

1981561 is a deficient number, since it is larger than the sum of its proper divisors (2939).

1981561 is a wasteful number, since it uses less digits than its factorization.

1981561 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 2938.

The product of its digits is 2160, while the sum is 31.

The square root of 1981561 is about 1407.6792958625. The cubic root of 1981561 is about 125.6037142058.

The spelling of 1981561 in words is "one million, nine hundred eighty-one thousand, five hundred sixty-one".