I don't understand Henry's approach. Here is mine.
sq=:#={:>:@-{.
a =: 2 4 5 6 12
b =: 1 2 3 4 5 6
c =: 1 12 13 13 14
seq =: 3 : 0
i =. # y
while. -. i +./@(sq\) y do. i =. <: i end.
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