Integer is Sum of Three Triangular Numbers/Historical Note

Historical Note on Integer is Sum of Three Triangular Numbers
proved that every Integer is Sum of Three Triangular Numbers.

The $18$th entry in his diary, dated $10$th July $1796$, made when he was $19$ years old, reads:
 * $**\Epsilon\Upsilon\Rho\Eta\Kappa\Alpha \quad \text{num} = \Delta + \Delta + \Delta.$