30

Number

$30$ (thirty) is:

$2 \times 3 \times 5$

$30 = 2 \times 3 \times 5$

$\dfrac 1 2 + \dfrac 1 3 + \dfrac 1 5 - \dfrac 1 {30} = 1$

The $1$st element of the $1$st triplet of consecutive positive even integers $n$ with the property $n + \map {\sigma_0} n = m$ for some $m$:

$30 + \map {\sigma_0} {30} = 32 + \map {\sigma_0} {32} = 34 + \map {\sigma_0} {34} = 38$

The area and perimeter of the $2$nd of the only $2$ Pythagorean triples which define a Pythagorean triangle whose area equals its perimeter:

$\tuple {5, 12, 13}$

The $3$rd primorial after $1$, $2$, $6$ (counting $1$ as the zeroth):

$30 = p_3 \# = 5\# = 2 \times 3 \times 5$

Hence the smallest positive integer with $3$ distinct prime factors

The $3$rd primorial which can be expressed as the product of consecutive integers:

$30 = 5 \# = 5 \times 6$

The $3$rd integer after $1$, $14$ whose divisor sum divided by its Euler $\phi$ value is a square:

$\dfrac {\map {\sigma_1} {30} } {\map \phi {30} } = \dfrac {72} 8 = 9 = 3^2$

The $4$th square pyramidal number after $1$, $5$, $14$:

$30 = 1 + 4 + 9 + 14 = \dfrac {4 \paren {4 + 1} \paren {2 \times 4 + 1} } 6$

The index (after $2$, $3$, $6$) of the $4$th Woodall prime:

$30 \times 2^{30} - 1$

The $4$th integer $n$ after $1$, $3$, $15$ with the property that $\map {\sigma_0} n \divides \map \phi n \divides \map {\sigma_1} n$:

$\map {\sigma_0} {30} = 8$, $\map \phi {30} = 8$, $\map {\sigma_1} {30} = 72$

The $5$th abundant number after $12$, $18$, $20$, $24$:

$1 + 2 + 3 + 5 + 6 + 10 + 15 = 42 > 30$

The $6$th positive integer $n$ after $5$, $11$, $17$, $23$, $29$ such that no factorial of an integer can end with $n$ zeroes

The $7$th semiperfect number after $6$, $12$, $18$, $20$, $24$, $28$:

$30 = 2 + 3 + 10 + 15$

The $7$th integer $m$ such that $m! - 1$ (its factorial minus $1$) is prime:

$3$, $4$, $6$, $7$, $12$, $14$, $30$

The number of integer partitions for $9$:

$\map p 9 = 30$

The $10$th and largest positive integer after $1$, $2$, $3$, $4$, $6$, $8$, $12$, $18$, $24$ such that all smaller positive integers coprime to it are prime

The $13$th highly abundant number after $1$, $2$, $3$, $4$, $6$, $8$, $10$, $12$, $16$, $18$, $20$, $24$:

$\map {\sigma_1} {30} = 72$

The $13$th of $35$ integers less than $91$ to which $91$ itself is a Fermat pseudoprime:

$3$, $4$, $9$, $10$, $12$, $16$, $17$, $22$, $23$, $25$, $27$, $29$, $30$, $\ldots$

The $14$th integer $n$ after $0$, $1$, $2$, $3$, $4$, $5$, $6$, $7$, $9$, $10$, $11$, $17$, $18$ such that $5^n$ contains no zero in its decimal representation:

$5^{30} = 931 \, 322 \, 574 \, 615 \, 478 \, 515 \, 625$

The $15$th after $1$, $2$, $3$, $4$, $5$, $6$, $7$, $8$, $10$, $12$, $14$, $16$, $18$, $24$ of $21$ integers which can be represented as the sum of two primes in the maximum number of ways

The $17$th harshad number after $1$, $2$, $3$, $4$, $5$, $6$, $7$, $8$, $9$, $10$, $12$, $18$, $20$, $21$, $24$, $27$:

$30 = 10 \times 3 = 10 \times \paren {3 + 0}$

The $19$th positive integer after $2$, $3$, $4$, $7$, $8$, $9$, $10$, $11$, $14$, $15$, $16$, $19$, $20$, $21$, $24$, $25$, $26$, $29$ which cannot be expressed as the sum of distinct pentagonal numbers.

The $22$nd (strictly) positive integer after $1$, $2$, $3$, $\ldots$, $20$, $21$, $24$, $25$, $26$, $27$ which cannot be expressed as the sum of distinct primes of the form $6 n - 1$