87

Number
$87$ (eighty-seven) is:


 * $3 \times 29$


 * The $2$nd positive integer after $1$ whose $\sigma$ value of its Euler $\phi$ value equals its $\sigma$ value:
 * $\map \sigma {\map \phi {87} } = \map \sigma {56} = 120 = \map \sigma {87}$


 * The $4$th after $4$, $13$, $38$ in the sequence formed by adding the squares of the first $n$ primes:
 * $87 = \displaystyle \sum_{i \mathop = 1}^4 {p_i}^2 = 2^2 + 3^2 + 5^2 + 7^2$


 * The $20$th lucky number:
 * $1$, $3$, $7$, $9$, $13$, $15$, $21$, $25$, $31$, $33$, $37$, $43$, $49$, $51$, $63$, $67$, $73$, $75$, $79$, $87$, $\ldots$


 * The $29$th semiprime:
 * $87 = 3 \times 29$


 * The $33$rd of $35$ integers less than $91$ to which $91$ itself is a Fermat pseudoprime:
 * $3$, $4$, $9$, $10$, $12$, $16$, $17$, $22$, $23$, $25$, $27$, $29$, $30$, $36$, $38$, $40$, $43$, $48$, $51$, $53$, $55$, $61$, $62$, $64$, $66$, $68$, $69$, $74$, $75$, $79$, $81$, $82$, $87$, $\ldots$

Also see