the number every four-digit number reaches in seven subtractions or fewer, unless all its digits are the same
Built as a memo rather than a table: the thing worth hanging it up for reads across the room, with the full reference in smaller type below it.
Three sizes in one archive — from a sheet on the fridge to a poster on the wall.
Leading zeros count as digits: a difference of 999 is written 0999. A number with four equal digits gives 0 after the first subtraction.
Dattatreya Ramchandra Kaprekar (1905–1986) taught at the government junior school in Devlali, Maharashtra, from 1930 to 1962. With no formal postgraduate training, he published extensively and became well known in recreational mathematics circles. Among his papers is “An interesting property of the number 6174”, and the constant now bears his name. In 1981 Prichett, Ludington and Lapenta showed that in base 10 only three- and four-digit numbers have such a constant: each of them whose digits are not all the same reaches the same number, 495 with three digits and 6174 with four.
Why the numbers can be trusted. Every countable number here is computed from data rather than typed in, and the sheet must pass 3 independent checks before it is allowed to build — how many items there are, the order they come in, and the claim in the highlighted box itself. If one fails, there is no file.
the IWF colour code and the plate count: from target weight to each side of the bar, no calculator
why a “yes” from a test that is right 99 times in 100 is wrong more than nine times in ten when one in a thousand has the thing
the first digit of a number: why a one turns up more than six times as often as a nine, and in which sets that is true
how many people it takes for two of them to share a birthday — and the same count for codes and fingerprints