217821681 has 96 divisors (see below), whose sum is σ = 420526080. Its totient is φ = 105235200.

The previous prime is 217821673. The next prime is 217821689. The reversal of 217821681 is 186128712.

It is an interprime number because it is at equal distance from previous prime (217821673) and next prime (217821689).

It is not a de Polignac number, because 217821681 - 2³ = 217821673 is a prime.

It is a super-2 number, since 2×217821681² = 94892569427331522, which contains 22 as substring.

It is not an unprimeable number, because it can be changed into a prime (217821689) by changing a digit.

It is a polite number, since it can be written in 95 ways as a sum of consecutive naturals, for example, 934741 + ... + 934973.

It is an arithmetic number, because the mean of its divisors is an integer number (4380480).

Almost surely, 2^217821681 is an apocalyptic number.

217821681 is a gapful number since it is divisible by the number (21) formed by its first and last digit.

It is an amenable number.

217821681 is a deficient number, since it is larger than the sum of its proper divisors (202704399).

217821681 is a wasteful number, since it uses less digits than its factorization.

217821681 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 347 (or 344 counting only the distinct ones).

The product of its digits is 10752, while the sum is 36.

The square root of 217821681 is about 14758.7831815499. The cubic root of 217821681 is about 601.6820219308.

The spelling of 217821681 in words is "two hundred seventeen million, eight hundred twenty-one thousand, six hundred eighty-one".