[magus]

Times Table

Theme: The numbers one, four, nine & sixteen in phrases
What the starred answers start with   PERFECTSQUARES  [are there imperfect ones?  ]

  Odd that both 11- and 12-Down are “layered,” both natural and both hard.
  Two of the four theme entries are from pop culture of the 1950’s --- my favorite time of Americana.  (Vivaldi was 200 years or so before that.)

RATING:    Three grins = Loved it; Two grins = Enjoyed it; One grin = A bit bland for my taste; One teardrop = Not much fun

[SJS]

Are there imperfect squares?

I think the term is used to distinguish a number that is a square of a whole number (perfect) vs. a square of some other number (I don't know if they are called imperfect though).  Most whole numbers appear to be squares of irrational numbers, like 2 is a square of the square root of two.  (Here irrational means "unable to be expressed as a ratio".)