1687991 has 2 divisors, whose sum is σ = 1687992. Its totient is φ = 1687990.

The previous prime is 1687969. The next prime is 1687999. The reversal of 1687991 is 1997861.

It is a happy number.

It is a strong prime.

It is a cyclic number.

It is not a de Polignac number, because 1687991 - 2¹⁰ = 1686967 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (1687999) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 843995 + 843996.

It is an arithmetic number, because the mean of its divisors is an integer number (843996).

2^1687991 is an apocalyptic number.

1687991 is a deficient number, since it is larger than the sum of its proper divisors (1).

1687991 is an equidigital number, since it uses as much as digits as its factorization.

1687991 is an evil number, because the sum of its binary digits is even.

The product of its digits is 27216, while the sum is 41.

The square root of 1687991 is about 1299.2270779198. The cubic root of 1687991 is about 119.0666246507.

The spelling of 1687991 in words is "one million, six hundred eighty-seven thousand, nine hundred ninety-one".