Graham&#x27;s number comes from such an extension, and is about 64 such operations deeper. It&#x27;s useful in the sense that it provides an upper bound for some proof that hasn&#x27;t yet been proven for ALL numbers.<p><a href="http:&#x2F;&#x2F;waitbutwhy.com&#x2F;2014&#x2F;11&#x2F;1000000-grahams-number.html" rel="nofollow">http:&#x2F;&#x2F;waitbutwhy.com&#x2F;2014&#x2F;11&#x2F;1000000-grahams-number.html</a>