Pentagonal and Hexagonal Numbers/Mistake
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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): $40,755$