What is 142857x7?

142857 times 7 is 999,999.

This is an interesting result because the six digits in the product are all 9's, and the sum of the digits is 54, which is a multiple of 9. Furthermore, if you add any pair of adjacent digits in the product, you get 9 (e.g. 9+9=18, 9+9=18, etc.).

This special property is related to the fact that 142857 is a repeating decimal with a period of 6:

1/7 = 0.142857142857...

When you multiply this repeating decimal by 7, you shift the digits to the left by one place, effectively "rotating" the digits of 142857:

142857 x 7 = 999,999.