217870921 has 2 divisors, whose sum is σ = 217870922. Its totient is φ = 217870920.

The previous prime is 217870903. The next prime is 217870937. The reversal of 217870921 is 129078712.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 215502400 + 2368521 = 14680^2 + 1539^2 .

It is a cyclic number.

It is not a de Polignac number, because 217870921 - 2⁹ = 217870409 is a prime.

It is a weakly prime.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 108935460 + 108935461.

It is an arithmetic number, because the mean of its divisors is an integer number (108935461).

Almost surely, 2^217870921 is an apocalyptic number.

It is an amenable number.

217870921 is a deficient number, since it is larger than the sum of its proper divisors (1).

217870921 is an equidigital number, since it uses as much as digits as its factorization.

217870921 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 14112, while the sum is 37.

The square root of 217870921 is about 14760.4512464897. The cubic root of 217870921 is about 601.7273565526.

It can be divided in two parts, 217870 and 921, that added together give a triangular number (218791 = T₆₆₁).

The spelling of 217870921 in words is "two hundred seventeen million, eight hundred seventy thousand, nine hundred twenty-one".