Minimum Quotient

Let m ( x ) m(x) be defined as the smallest number that can be evenly divided by all the positive integers between 1 1 and x x inclusive.

(For example, m ( 4 ) = 12 m(4) = 12 because 12 12 is the smallest number that can be evenly divided by 1 1 , 2 2 , 3 3 , and 4 4 .)

Find the value of m ( 130 ) m ( 120 ) \frac{m(130)}{m(120)} .


The answer is 13970.

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3 solutions

David Vreken
Apr 11, 2018

The value of m ( x ) m(x) can be found by multiplying all the prime numbers less than or equal to x x , and raising each prime number p p to the highest exponent n n such that p n x p^n \leq x .

Therefore,

m ( 120 ) = 2 6 3 4 5 2 7 2 11 13 113 m(120) = 2^6 \cdot 3^4 \cdot 5^2 \cdot 7^2 \cdot 11 \cdot 13 \dots 113

m ( 130 ) = 2 7 3 4 5 3 7 2 1 1 2 13 113 127 m(130) = 2^7 \cdot 3^4 \cdot 5^3 \cdot 7^2 \cdot 11^2 \cdot 13 \dots 113 \cdot 127

So m ( 130 m ( 120 ) = 2 7 3 4 5 3 7 2 1 1 2 13 113 127 2 6 3 4 5 2 7 2 11 13 113 = 2 5 11 127 = 13970 \frac{m(130}{m(120)} = \frac{2^7 \cdot 3^4 \cdot 5^3 \cdot 7^2 \cdot 11^2 \cdot 13 \dots 113 \cdot 127}{2^6 \cdot 3^4 \cdot 5^2 \cdot 7^2 \cdot 11 \cdot 13 \dots 113} = 2 \cdot 5 \cdot 11 \cdot 127 = \boxed{13970} .

Giorgos K.
Apr 11, 2018

M a t h e m a t i c a Mathematica

LCM@@Range@130/LCM@@Range@120

returns 13970 13970

Very concise solution!

David Vreken - 3 years, 2 months ago
Naren Bhandari
Apr 11, 2018

The smallest number that evenly divides m ( x ) m(x) is the lcm \text{lcm} from 1 1 to x x and lcm(1.2.3,...,x) \text{lcm(1.2.3,...,x)} is the product of all primes numbers less than or equal x x ie lcm(1 to x ) \text{lcm(1 to x )} is ( p 1 q 1 x ) ( p 2 q 2 x ) ( p 3 q 3 x ) ( p n q n x ) \small (p_1^{q_1}\leq x)\cdot\,(p_2^{q_2}\leq x)\cdot(p_3^{q_3}\leq x)\cdots (p_n^{q_n}\leq x )

lcm(1,2,3,....,130) lcm(1,2,3,4,.....120) = 2 7 3 4 5 3 7 2 1 1 2 2 6 3 4 5 2 7 2 11 ( 13 17 23 113 13 17 23 113 ) 127 m ( 130 ) m ( 120 ) = 2 5 11 127 = 13970 \therefore \dfrac{\text{lcm(1,2,3,....,130)}}{\text{lcm(1,2,3,4,.....120)}} = \dfrac{2^7\cdot 3^4\cdot 5^3\cdot 7^2\cdot 11^2}{2^6\cdot 3^4 \cdot 5^2 \cdot 7^2 \cdot 11}\left(\dfrac{13\cdot 17\cdots 23\cdots 113}{13\cdot 17\cdots23 \cdots 113}\right) \cdot 127 \\ \implies \dfrac{m(130)}{m(120)} = 2\cdot 5\cdot 11\ 127 = 13970


Note: In case of lcm ( 1 , 2 , 3 , . . . 130 ) \text{lcm}(1,2,3,...130) , 128 ( 2 7 ) , 125 ( 5 3 ) , 81 ( 3 4 ) , 49 ( 7 2 ) , 121 ( 1 1 2 ) 128(2^7) , 125(5^3),81(3^4),49(7^2), 121(11^2) appears multiple times and primes between 11 and 130 appears only a time for composite numbers between them. On the same way lcm from 1 to 120 can also be determined.

Nice solution!

David Vreken - 3 years, 2 months ago

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Thank you !:)

Naren Bhandari - 3 years, 2 months ago

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