Pentagonal and Hexagonal Numbers/Mistake

Source Work

1997: David Wells: Curious and Interesting Numbers (2nd ed.):

The Dictionary

$40,755$

Mistake

The first number, after $1$ and $15$, to be simultaneously pentagonal and hexagonal and therefore, also, triangular.

Correction

$15$ is not a pentagonal number as such.

It is, however, a second pentagonal number, and so is classified as a generalized pentagonal number.

If this is what David Wells meant, then he should have stated it.

Sources

• 1997: David Wells: Curious and Interesting Numbers (2nd ed.) ... (previous) ... (next): $40,755$