The sum of ten, not necessarily different, positive integers is 100. If placed adequately on the vertices of this graph, two of them will be joined by a line if, and only if, they have a common divisor greater than 1 (i.e. they are not relatively prime). enter image description here

What are those ten integers?


2 Answers 2


Start by labeling as follows:

enter image description here

Solution process:

The only pairs of integers that could possibly be equal are $U$ and $X$, and $W$ and $Z$. This is because if two integers are equal to each other then they're certainly relatively prime to the same integers. Thus, for example, $Q \ne R$ because $R$ and $Y$ are not relatively prime but $Q$ and $Y$ are.

Unfortunately we can't automatically deduce that $U = X$ or $W = Z$. It may be possible that, without loss of generality, $U = X^k$ for some integer $k$, with a similar possible relationship between $W$ and $Z$.

Because $U$ and $X$ are only joined to each other, I suspect that each has only one prime factor (that is, each is some power of the same prime). Theoretically it's possible that, say, $U = pq$ for two distinct primes $p$ and $q$, but then no other vertex aside from $X$ can have $p$ or $q$ as a factor, and that seems very limiting since we're trying to sum to $100$.

I started by building systematically and had to adjust as I went along. My graph theory and number theory skills are rusty so if there's a more systematic way that doesn't involve guess-and-check, I don't know what it is.

First, $S,T,Y$ are all adjacent to each other (adjacent = "joined by an edge" in graph theory terms [edge = "line segment" in graph theory terms]), so $S,T,Y$ must have at least one prime factor in common. This common factor is $3$.

Since $S$ is adjacent to $V$, and $T$ and $Y$ are not, then $S$ must have a prime factor that $T$ and $Y$ do not. This factor is $7$.

Also, $Y$ is adjacent to $R$, while $S$ and $T$ are not. Thus $Y$ must have a prime factor that $S$ and $T$ do not. This factor is $2$.

$R$ must have another prime factor that no other vertex has yet, since $R$ is adjacent to $Q$ and no other vertex is adjacent to $Q$. This factor is $5$.

If we assume the simplest case, i.e., no powers of primes and just the primes themselves, then we have $Q = 5, R = 10, S = 21, T = 3, V = 7, Y = 6.$

Thus the total sum so far is $5+10+21+3+7+6 = 52$. This means we need $U+X+W+Z = 48$. What luck! $48 = 22 + 26 = 11 + 11 + 13 + 13$. Take $U = X = 13$ and $W = Z = 11$ and we get one possible answer:

enter image description here

  • $\begingroup$ The answer is unique. $\endgroup$ Commented May 18, 2017 at 12:18
  • $\begingroup$ @BernardoRecamánSantos, up to isomorphism. :P $\quad$ $\endgroup$
    – user33097
    Commented May 18, 2017 at 12:25
  • $\begingroup$ The partition of 100 is unique. No other partition of 100 has the same graph. $\endgroup$ Commented May 18, 2017 at 13:54
  • $\begingroup$ @BernardoRecamánSantos, I know, I was thinking isomorphism because the $11$s and $13$s could be swapped, thereby having different labels. But I forgot that (a) I provided the labels and (b) they're irrelevant anyway since a graph is just vertices and edges. $\endgroup$
    – user33097
    Commented May 18, 2017 at 13:59

If I understand correctly, there should be many solutions to this. Then again, English is not my native language. Clockwise, starting from the topmost left: 26, 16, 8, 8, 10, 10, 10, 2, 8, 2.

I assume that I am wrong since this seems very easy, so please do correct me and explain if I haven't understood.

  • 1
    $\begingroup$ I think that's not right because $26$ and $8$ aren't joined by an edge, which, according to the rules, means they shouldn't have any common factors greater than $1$. But they do ($2$). $\endgroup$
    – user33097
    Commented May 17, 2017 at 13:11
  • $\begingroup$ Ah! Correct, they should be joined. This is what I have overlooked, that lines should not be static! I was approaching the graph in an incorrect way. Thank you! Please downvote and flag this answer. :) $\endgroup$
    – BadMathMan
    Commented May 17, 2017 at 13:12
  • 3
    $\begingroup$ You can always delete your own answer if you want... $\endgroup$
    – Mithical
    Commented May 17, 2017 at 13:14
  • 1
    $\begingroup$ Meh, I largely find downvoting to be petty and almost never do it. Also, what Mithrandir said. $\endgroup$
    – user33097
    Commented May 17, 2017 at 13:17
  • $\begingroup$ But I'll flag so I can get progress towards a badge I'll never earn. :P $\quad$ $\endgroup$
    – user33097
    Commented May 17, 2017 at 13:18

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