#Java 8, 2620 bytes
(int n)->Math.sqrt(n)%1==0
(or 20 bytes, Input is an still debatable ):int.
n->Math.sqrt(n)%1==0
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(int n)->Math.sqrt(n)%1==0
(or 20 bytes, Input is an still debatable ):int.
n->Math.sqrt(n)%1==0