1581 has 8 divisors (see below), whose sum is σ = 2304. Its totient is φ = 960.

The previous prime is 1579. The next prime is 1583. The reversal of 1581 is 1851.

Adding to 1581 its sum of digits (15), we get a triangular number (1596 = T_{56}).

1581 = 9^{2} + 10^{2} + ... + 17^{2}.

It is a happy number.

It is an interprime number because it is at equal distance from previous prime (1579) and next prime (1583).

It is a sphenic number, since it is the product of 3 distinct primes.

It is not a de Polignac number, because 1581 - 2^{1} = 1579 is a prime.

It is a super-2 number, since 2×1581^{2} = 4999122, which contains 22 as substring.

It is a Smith number, since the sum of its digits (15) coincides with the sum of the digits of its prime factors. Since it is squarefree, it is also a hoax number.

It is a Curzon number.

It is a self number, because there is not a number *n* which added to its sum of digits gives 1581.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (1583) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 36 + ... + 66.

It is an arithmetic number, because the mean of its divisors is an integer number (288).

It is an amenable number.

1581 is a deficient number, since it is larger than the sum of its proper divisors (723).

1581 is a wasteful number, since it uses less digits than its factorization.

1581 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 51.

The product of its digits is 40, while the sum is 15.

The square root of 1581 is about 39.7617907041. The cubic root of 1581 is about 11.6495895279.

The spelling of 1581 in words is "one thousand, five hundred eighty-one".

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