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I came across this symbol on page xix of the book Universal Artificial Intelligence by Hutter:

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(link to full text of book)

It is used for the Solomonoff-Levin universal semi-measure. I've never seen it before, and wondering if anyone can help me understand. Thanks.

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It is specified on that very same page:

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Thus, $a\overset{\times}{=}b$ means "equal, within a multiplicative constant".

More information is on p.33:

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  • $\begingroup$ Thank you Zev. I wish I had seen that before posting the question. $\endgroup$ Commented Jun 11, 2015 at 2:36
  • $\begingroup$ Would that be $a=c \cdot b$? $\endgroup$ Commented Jun 11, 2015 at 2:36
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    $\begingroup$ @YoTengoUnLCD: I've updated my answer. In particular, $a$ and $b$ are intended to be functions, not constants, so $a\overset{\times}{=}b$ means $a=O(b)$ and $b=O(a)$. $\endgroup$ Commented Jun 11, 2015 at 2:42
  • $\begingroup$ $a = \Theta(b)$ is the more usual notation for what's described here as $a\overset{\times}{=}b$ (at least, for $a$ and $b$ positive, etc.) $\endgroup$
    – anomaly
    Commented Jan 9, 2016 at 3:30

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