To prove that the bus number cannot be any smaller than 12 requires an exhaustive search. No number strictly less than 12 can be written c1 + ... + ck in two different ways such that the products c1c2...ck are equal. Also the only two ways this can be done for 12 are 1,3,4,4 and 2,2,2,6 as given above. This is easily checked by computer, but if anyone has a better argument, please let me know.