Smaller of Thabit Pair is Tetrahedral
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Theorem
Let $\tuple {m_1, m_2}$ be a Thabit pair such that $m_1 < m_2$.
Then $m_1$ is a tetrahedral number.
Proof
By Thabit's Rule:
\(\ds m_1\) | \(=\) | \(\ds 2^n \times \paren {3 \times 2^{n - 1} - 1} \times \paren {3 \times 2^n - 1}\) | ||||||||||||
\(\ds m_2\) | \(=\) | \(\ds 2^n \times \paren {9 \times 2^{2 n - 1} - 1}\) |
for some $n \in \Z_{\ge 0}$.
We have that:
\(\ds \paren {3 \times 2^{n - 1} - 1} \times \paren {3 \times 2^n - 1}\) | \(=\) | \(\ds 9 \times 2^{2 n - 1} - 3 \times 2^n - 3 \times 2^{n - 1} + 1\) | ||||||||||||
\(\ds \) | \(<\) | \(\ds 9 \times 2^{2 n - 1} - 1\) |
and so $m_1 < m_2$.
Then:
\(\ds m_1\) | \(=\) | \(\ds 2^n \times \paren {3 \times 2^{n - 1} - 1} \times \paren {3 \times 2^n - 1}\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \frac {2^n \times \paren {3 \times 2^{n - 1} - 1} \times \paren {3 \times 2^n - 1} \times 6} 6\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \frac {\paren {3 \times 2^n} \times \paren {2 \times 3 \times 2^{n - 1} - 2} \times \paren {3 \times 2^n - 1} } 6\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \frac {\paren {3 \times 2^n} \times \paren {3 \times 2^n - 1} \times \paren {3 \times 2^n - 2} } 6\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \frac {k \times \paren {k + 1} \times \paren {k + 2} } 6\) | where $k = 3 \times 2^n - 2$ |
From Closed Form for Tetrahedral Numbers, this is the $k$th tetrahedral number where $k = 3 \times 2^n - 2$.
$\blacksquare$
Sources
- 1986: David Wells: Curious and Interesting Numbers ... (previous) ... (next): $220$
- 1997: David Wells: Curious and Interesting Numbers (2nd ed.) ... (previous) ... (next): $220$