# Square Pyramidal and Triangular Numbers

## Theorem

The only positive integers which are simultaneously square pyramidal and triangular are:

$1, 55, 91, 208 \, 335$

## Proof

 $\displaystyle 1$ $=$ $\displaystyle \dfrac {1 \paren {1 + 1} \paren {2 \times 1 + 1} } 6$ Closed Form for Square Pyramidal Numbers $\displaystyle$ $=$ $\displaystyle \dfrac {1 \times \paren {1 + 1} } 2$ Closed Form for Triangular Numbers

 $\displaystyle 55$ $=$ $\displaystyle \dfrac {5 \paren {5 + 1} \paren {2 \times 5 + 1} } 6$ Closed Form for Square Pyramidal Numbers $\displaystyle$ $=$ $\displaystyle \dfrac {10 \times \paren {10 + 1} } 2$ Closed Form for Triangular Numbers

 $\displaystyle 91$ $=$ $\displaystyle \dfrac {6 \paren {6 + 1} \paren {2 \times 6 + 1} } 6$ Closed Form for Square Pyramidal Numbers $\displaystyle$ $=$ $\displaystyle \dfrac {13 \times \paren {13 + 1} } 2$ Closed Form for Triangular Numbers

 $\displaystyle 208 \, 335$ $=$ $\displaystyle \dfrac {85 \paren {85 + 1} \paren {2 \times 85 + 1} } 6$ Closed Form for Square Pyramidal Numbers $\displaystyle$ $=$ $\displaystyle \dfrac {645 \times \paren {645 + 1} } 2$ Closed Form for Triangular Numbers