[[18]] is not prime (composite). Want proof? :)

Full prime factorization: [[2]] * [[3]] * [[3]]

Factor pairs:

[[18]] = [[2]] * [[9]]

[[18]] = [[3]] * [[6]]

Proper divisors: [[2]], [[3]], [[6]], [[9]]

Primes up to 18: [[2]], [[3]], [[5]], [[7]], [[11]], [[13]], [[17]].

[[18]] is not a centered hexagonal number.

The closest centered hexagonal numbers are [[7]] (index # [[2]]) and [[19]] (index # [[3]]).

First 20 centered hexagonal numbers: [[1]], [[7]], [[19]], [[37]], [[61]], [[91]], [[127]], [[169]], [[217]], [[271]], [[331]], [[397]], [[469]], [[547]], [[631]], [[721]], [[817]], [[919]], [[1027]], [[1141]].

[[hz/18]] is closest to [[D0]] at [[18]] Hz (-34 cents).

Note class: [[D]], Octave: [[octave/0]].

• [[square]]: 324

• [[square root]]: 4.2426

• [[cube]]: 5832

• [[cube root]]: 2.6207

• [[natural log]] (ln): 2.8904

• [[base-10 log]] (log10): 1.2553

• [[base-2 log]] (log2): 4.1699