Number Gossip

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Unique Properties of 31

  • 31 is the earliest and the only known case such that the sum of the divisors of two distinct numbers (16 & 25) is the same prime quantity (31), that is to say: 1+2+4+8+16 = 31 and 1+5+25 = 31
  • Given six points on a circle connected by lines, the maximum number of regions is 31. This answer is famous for breaking a pattern as if there were fewer points, the number of regions would be a power of 2
  • 31 is the number of minimal composites which cover the set of composites in base 10
  • There are only 31 numbers which cannot be expressed as the sum of distinct squares
  • There are only 31 numbers which when multiplied by any power of 4 are not the sums of 4 distinct squares
  • 31 = 22 + 33, i.e., the sum of the first two primes raised to themselves
  • 31 is the smallest prime that is a sum of two triangular numbers with prime indices
  • 31 is the smallest prime p such that neither 6p-1 nor 6p+1 are primes

Rare Properties of 31

Mersenne prime

Common Properties of 31