4961
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Number
$4961$ (four thousand, nine hundred and sixty-one) is:
- $11^2 \times 41$
- The $18$th Fermat pseudoprime to base $3$ after $91$, $121$, $286$, $671$, $703$, $949$, $1105$, $1541$, $1729$, $1891$, $2465$, $2665$, $2701$, $2821$, $3281$, $3367$, $3751$:
- $3^{4961} \equiv 3 \pmod {4961}$
- The $41$st octagonal number, after $1$, $8$, $21$, $40$, $65$, $\ldots$, $3008$, $3201$, $3400$, $3605$, $3816$, $4033$, $4256$, $4485$, $4720$:
- $4961 = \ds \sum_{k \mathop = 1}^{41} \paren {6 k - 5} = 41 \paren {3 \times 41 - 2}$