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

Full prime factorization: [[3]] * [[7]]

Factor pairs:

[[21]] = [[3]] * [[7]]

Proper divisors: [[3]], [[7]]

Primes up to 21: [[2]], [[3]], [[5]], [[7]], [[11]], [[13]], [[17]], [[19]].

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

The closest centered hexagonal numbers are [[19]] (index # [[3]]) and [[37]] (index # [[4]]).

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/21]] is closest to [[E0]] at [[21]] Hz (+33 cents).

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

• [[square]]: 441

• [[square root]]: 4.5826

• [[cube]]: 9261

• [[cube root]]: 2.7589

• [[natural log]] (ln): 3.0445

• [[base-10 log]] (log10): 1.3222

• [[base-2 log]] (log2): 4.3923