Do we share the prime cluster?
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rÆPE


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How it works

rÆPE    Main link. Arguments: N, M
r       Yield the range of integers between N and M, inclusive.
 ÆP     For each integer, yield 1 if it is prime, 0 otherwise.
   E    Yield 1 if all items are equal (none in the range were prime,
        or there's only one item).


Works because two numbers have different prime clusters iff there is a prime between them, or either number is itself prime; unless both numbers are the same, in which case E returns 1 anyway (all items in a single-item array are equal).