861 is the 41st triangular number.
41 + 40 + 39 + … + 3 + 2 +1 = 861
Two quick ways to compute a selected triangular number:
41 x 21 = 861
41 + 1 = 42
41/2 = 20.5
42 x 20.5 = 861
          862
  Factors into 431 * 2
  Notice that the digits 1 through 4 are used just once.
          863
  The 150th Prime Number
          864
  Factors neatly into: 2[sup]5[/sup] * 3[sup]3[/sup]
  = 32 * 27
          865
  =2² +3² +4² +5² +6² +6³ +7² +8² +9² +10² +11² +12²
  =4 + 9 + 16 + 25 +36 + 216 +49 +64 +81 +100 +121 + 144
          867
  17² * 3
  = 289 * 3
          868
  Another neat factorization 2 * 2 * 7 * 31
          869
  Factors into two 2 digit prime numbers 11 * 79
          870
  This is the product of 2 consecutive numbers
  870 = 29 * 30
          871
  871 is the product of 2 two digit prime numbers:
  871 = 13 * 67
Photos of a home at 872 Amanda Drive, James Island
          873
  The sum of a perfect square and cube
  = 12[sup]2[/sup] + 9[sup]3[/sup]
  = 144 + 729
          874
  Factors into 2 * 19 * 23
          875
  Another interesting factorization:
  875 = 5[sup]3[/sup] * 7
  = 5 * 5 * 5 * 7
  = 25 * 35
Louis the German, King of East Francia, died in the year 876 AD.
          877
  A prime number (the 151st to be exact)
Studio 878 provides dance lessons to the people of Cape Cod.
Dean Smith’s 879 wins make him the winningest college basketball coach ever.
There are 880 4 x 4 magic squares.