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==Explicit formulas==
Every even number can be written in the form 9n, and every odd number can be written in the form 9n+1.
Write {{math|''N''<sub>''k''</sub>}} for the {{mvar|k}}th square triangular number, and write {{math|''s''<sub>''k''</sub>}} and {{math|''t''<sub>''k''</sub>}} for the sides of the corresponding square and triangle, so that
:<math>N_k = s_k^2 = \frac{t_k(t_k+1)}{2}.</math>
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