Number Gossip

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Unique Properties of 15

  • 15 is the smallest emirpimes
  • 15 is the smallest composite cyclic number, that is number n with the property that there is only one group of order n
  • 15 is the magic constant of the unique order-3 normal magic square
  • 15 is the smallest number that equals the sum of numbers between its first and the second digits inclusive
  • There are exactly 15 primes that are both left-truncatable and right-truncatable (deleting any number of digits at left or at right leaves a prime)
  • 15 is the smallest product of twin primes
  • The 15 theorem states that if a positive definite quadratic form with integer matrix represents all positive integers up to 15, then it represents all positive integers
  • 15 is the only composite odd teenage number (submitted by Isaac LaComb)

Rare Properties of 15


Common Properties of 15