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This Python program uses some useful string functions from the

enigma.pylibrary. It runs in 80ms.Solution:The maximum possible number of bulbs is 65,821 (= 7 × 9403).There seem to be remarkably few numbers of bulbs > 10000 that leave us with no repeated digit,

at least when planted in a long narrow strip of 2, 3, 5, or 7 rows.

I found only 17082 = 3 × 5694, 20457 = 3 × 6819, 20754 = 3 × 6918,

21658 = 7 × 3094, 24507 = 3 × 8169, 27504 = 3 × 9168, 28651 = 7 × 4093,

65128 = 7 × 9304, 65821 = 7 × 9403.

Of those, only 4093 and 9403 are prime.

I haven’t yet tested 2 digits times 3 digits.