Maxlcm2

Four distinct positive integers a , b , c , d a, b, c, d satisfy a + b + c + d = 1002. a+b+c+d=1002. Find the maximum value of lcm ( a , b , c , d ) . \text{lcm}(a,b,c,d).


Inspiration


The answer is 3935249535.

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1 solution

Yvonne Killian
Aug 10, 2018

To maximize the lcm of the integer variables a, b, c, d with a given sum p,

  • the values chosen for a, b, c, d should be coprime

  • the values chosen for a, b, c, d should be close to p/4.

Here, p = a + b + c + d = 1002, so the values chosen for a, b, c, d must all be odd values, close to 250.5.

In the range 244 - 257, the coprimes are

  • 247 (13x19), 251 (prime), 253 (11x23) and 257 (prime)

  • either 245 (5x7^2) or 255 (3x5x17)

  • either 249 (3x83) or 255 (3x5x17).

There are two combinations of these values which add up to 1002. These combinations are, together with their lcm's:

  • 245, 247, 253, 257 (3934745815)

  • 245, 249, 251, 257 (3935249535)

So the solution is 3935249535 (not surprisingly, as 249 and 251 are closer to 250.5 than 247 and 253).

Oops,249 and 255 are both mutiples of 3,so I think the answer is still 3935249535

X X - 2 years, 10 months ago

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Thank you! I changed my solution (twice, and I think it's OK now...)

Yvonne Killian - 2 years, 10 months ago

The answer is correct.

Maurice van Peursem - 2 years, 10 months ago

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