Split 2018 Ⅱ

Find the minimum value of n n such that the sum of positive integers a 1 , a 2 , , a n a_1,a_2,\ldots,a_n is 2018 and any positive integer less than or equal to 2018 is the sum of some of these integers.


Bonus: Generalize this for the sum of integer N . N.


The answer is 11.

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2 solutions

Otto Bretscher
Dec 18, 2018

Ten summands are not enough, since we can form at most 2 10 = 1024 2^{10}=1024 distinct sums out of them. But 11 works: We can write 2018 = 995 + 512 + 256 + 128 + 64 + 32 + 16 + 8 + 4 + 2 + 1 2018=995+512+256+128+64+32+16+8+4+2+1

Any positive integer n < 1024 n<1024 can be written uniquely as a sum of powers of 2, up to 512, and any n n with 2018 n 1024 2018 \geq n\geq 1024 can be written as n = 995 + ( a sum of powers of 2 up to 512 ) n=995+(\text{a sum of powers of 2 up to 512}) . Thus the answer is 11 \boxed{11} .

In general, n n is the number of digits of N N in base 2.

A charming problem that I have never seen in this form before. Thank you!

It's inspired by this problem .

Brian Lie - 2 years, 5 months ago

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Very true! I missed that one; I wish I had the time to look at all these problems...

Otto Bretscher - 2 years, 5 months ago
Prince Zarzees
Dec 21, 2018

Just think about binary numbers, a 11 bit binary number can represent 0 to 2047

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