[[81]] is not prime (composite): [[3]] * [[3]] * [[3]] * [[3]]

Factor pairs: [[3]] * [[27]], [[9]] * [[9]]

Prime neighborhood: [[71]], [[73]], [[79]], [[81]], [[83]], [[89]], [[97]]

Prime gap: 4 (interval [[79]] to [[83]])

Primes up to 81: [[2]], [[3]], [[5]], [[7]], [[11]], [[13]], [[17]], [[19]], [[23]], [[29]], [[31]], [[37]], [[41]], [[43]], [[47]], [[53]], [[59]], [[61]], [[67]], [[71]], [[73]], [[79]] (22 total).

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

The closest centered hexagonal numbers are [[61]] (index # [[5]]) and [[91]] (index # [[6]]).

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/81]] is closest to [[E2]] at [[81]] Hz (-30 cents).

Note class: [[E]], Octave: [[octave/2]].

• [[square]]: 6561

• [[square root]]: 9.0000

• [[cube]]: 531441

• [[cube root]]: 4.3267

• [[natural log]] (ln): 4.3944

• [[base-10 log]] (log10): 1.9085

• [[base-2 log]] (log2): 6.3399