FORBIDDEN NUMBER 3 / 21
PHYSICALLY UNREACHABLE [CLASSIFIED]
Graham's Number
gā‚†ā‚„ ā€” so large it cannot be expressed with conventional notation, only with Knuth's up-arrow towers iterated 64 times
WHY IT IS FORBIDDEN
Not forbidden by law, but forbidden by physics: there are not enough particles in the observable universe to write its digits, nor enough time before heat death to compute them.
Used by Ronald Graham as an upper bound in Ramsey theory (1971). For decades it was the largest number ever used in a serious mathematical proof.
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