a ppt is here

with lots of subtracting to be done

and the tasks provide a sensible reason for introducing algebra (unlike life in general...)

start with any four, smallish, numbers - in any order, initially

calculate the positive difference between an adjacent pair of numbers, looping back to the start for the right-hand end number, to produce a new set of 4 numbers

keep on doing this, line by line

until you have a good reason to stop

Herbert Wills analysed this task in 1971 and gave it the name 'Diffy'

the work was brought to my attention by a fine booklet from 'Motivated Math Project' by Stanley Bezuska at Boston College Mathematics Institute (published in 1976, I think)

you can return to the basic task of doing a 'Diffy', year on year, with fresh and increasingly complicated starting four numbers - chosen as consecutive terms from various 'standard' number patterns (see below)

various algebraic skills can be practised for ever more complicated number patterns to establish a generalisation for the number of steps it always seems to take to reach 0 0 0 0

all starting arrangements of four numbers reduce to 0 0 0 0, quite quickly in most cases - usually in fewer than 7 steps

it's easy to make errors and tedious to check, so it can be helpful to set up a spreadsheet in advance

(using abs(difference between cells))

to begin the task(s):

ask students for any 4 numbers (not too big and not in any order) and then go through the 'diffy' process, without explanation - they try to sort out what the rules for constructing next lines are...

it is quite hard to find a set of numbers that involves more than six steps (iterations)

but it is possible

here are two examples

start to input four consecutive terms of a sequence and explore what happens

e.g. for a constant difference pattern:

- try out several particular examples
- see what patterns are common to all the examples (or a few at least)
- decide how many steps a 'diffy' seems to take for a particular number pattern
- prove this using algebra

- consecutive multiples (start with a number keep multiplying by e.g. 2)
- a linear rule: start with a number, multiply by e.g. 3 and e.g. subtract 2 each time
- consecutive fibonacci numbers
- consecutive square numbers
- consecutive triangular numbers
- consecutive cubes
- consecutive terms of a general geometric sequence

these are all included on a powerpoint

Puntmat have an interesting variation of this task, using the NLVM interactive square, asking students to find a sequence of particular numbers after four iterations (steps)

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