15781561 has 2 divisors, whose sum is σ = 15781562. Its totient is φ = 15781560.

The previous prime is 15781543. The next prime is 15781583. The reversal of 15781561 is 16518751.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 10112400 + 5669161 = 3180^2 + 2381^2 .

It is a cyclic number.

It is not a de Polignac number, because 15781561 - 2⁵ = 15781529 is a prime.

It is not a weakly prime, because it can be changed into another prime (15781541) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 7890780 + 7890781.

It is an arithmetic number, because the mean of its divisors is an integer number (7890781).

Almost surely, 2^15781561 is an apocalyptic number.

It is an amenable number.

15781561 is a deficient number, since it is larger than the sum of its proper divisors (1).

15781561 is an equidigital number, since it uses as much as digits as its factorization.

15781561 is an evil number, because the sum of its binary digits is even.

The product of its digits is 8400, while the sum is 34.

The square root of 15781561 is about 3972.6012888283. The cubic root of 15781561 is about 250.8322185749.

It can be divided in two parts, 157 and 81561, that added together give a palindrome (81718).

The spelling of 15781561 in words is "fifteen million, seven hundred eighty-one thousand, five hundred sixty-one".