#math trivia #169 solution

Recall that “day-numbers” are integers between 1 and 366 — the numbers of the days of the year (366 is included because the problem was given during a leap year, though that won’t affect the result).

The largest square between 1 and 366 is 192 = 361, so all the answers must be squares of a prime less than or equal to 19.  There are eight such primes, and consequently eight acceptable day-numbers:

  • 1 = 12
  • 4 = 22
  • 9 = 32
  • 25 = 52
  • 49 = 72
  • 121 = 112
  • 169 = 132
  • 289 = 172
  • 361 = 192

Note:  The number of prime squares less than or equal to a limit N is approximately 2 sqrt(N) / ln(N), where ln is the natural logarithm.  For N = 366, the value is about 6.48, which is reasonably close to the actual answer 8.  The estimate is a consequence of the Prime Number Theorem, which states that the number of primes less than or equal to a limit N is approximately N / ln(N).  Because we’re looking for prime squares, our estimate is based on the number of primes less than or equal to the square root of the limit, which is sqrt(N) / ln(sqrt(N)) = sqrt(N) / (ln(N) / 2) = 2 sqrt(N) / ln(N).

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