87
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Number
$87$ (eighty-seven) is:
- $3 \times 29$
- The $2$nd positive integer after $1$ whose divisor sum of its Euler $\phi$ value equals its divisor sum:
- $\map {\sigma_1} {\map \phi {87} } = \map {\sigma_1} {56} = 120 = \map {\sigma_1} {87}$
- The $4$th after $4$, $13$, $38$ in the sequence formed by adding the squares of the first $n$ primes:
- $87 = \ds \sum_{i \mathop = 1}^4 {p_i}^2 = 2^2 + 3^2 + 5^2 + 7^2$
- The $4$th positive integer after $1$, $24$, $26$ whose Euler $\phi$ value is equal to the product of its digits:
- $\map \phi {87} = 56 = 8 \times 7$
- 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:
- $3$, $4$, $9$, $10$, $12$, $16$, $17$, $22$, $23$, $25$, $27$, $29$, $30$, $36$, $38$, $40$, $43$, $48$, $51$, $53$, $55$, $61$, $62$, $64$, $66$, $68$, $69$, $74$, $75$, $79$, $81$, $82$, $87$, $\ldots$
Arithmetic Functions on $87$
\(\ds \map \phi { 87 }\) | \(=\) | \(\ds 56\) | $\phi$ of $87$ | |||||||||||
\(\ds \map {\sigma_1} { 87 }\) | \(=\) | \(\ds 120\) | $\sigma_1$ of $87$ |
Also see
- Previous ... Next: Integers for which Divisor Sum of Phi equals Divisor Sum
- Previous ... Next: Numbers for which Euler Phi Function equals Product of Digits
- Previous ... Next: Sum of Sequence of Squares of Primes
- Previous ... Next: Lucky Number
- Previous ... Next: 91 is Pseudoprime to 35 Bases less than 91
- Previous ... Next: Semiprime Number
Sources
- 1997: David Wells: Curious and Interesting Numbers (2nd ed.) ... (previous) ... (next): $87$