1541
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Number
$1541$ (one thousand, five hundred and forty-one) is:
- $23 \times 67$
- The $6$th Fermat pseudoprime to base $5$ after $4$, $124$, $217$, $561$, $781$:
- $5^{1541} \equiv 5 \pmod {1541}$
- The $8$th Fermat pseudoprime to base $3$ after $91$, $121$, $286$, $671$, $703$, $949$, $1105$:
- $3^{1541} \equiv 3 \pmod {1541}$
- The $23$rd octagonal number, after $1$, $8$, $21$, $40$, $65$, $\ldots$, $645$, $736$, $833$, $936$, $1045$, $1160$, $1281$, $1408$:
- $1541 = \ds \sum_{k \mathop = 1}^{23} \paren {6 k - 5} = 23 \paren {3 \times 23 - 2}$