HP 40gs HP 39gs_40gs_Mastering The Graphing Calculator_English_E_F2224-90010.p - Page 100

If the definition is recursive but only involves Tn−1 rather than both Tn−1 and Tn−2 then you need not enter a value for U1(2). For example, for the sequence Tn = Tn−1 + 3; T1 = 2 you need only enter the value 5 into U1(1) and the expression U1(N-1)+4 into U1(N). The value of U1(2) will be ignored in the SYMB view but filled in by the calculator automatically in the NUM view. Convenient screen keys provided There are a number of very convenient extra buttons provided at the bottom of the screen when entering sequences. Two of these - and - are available as soon as the cursor moves onto the U(N) line (see right). Pressing either will enter the appropriate text into the sequence definition. The rest become visible once you have begun to enter the sequence definition. For example, suppose we enter the Fibonacci sequence into U2 by defining U2(N) as U2(N-1) + U2(N-2). Rather than having to type all of this we can use the buttons provided, pressing: This is a very convenient feature, and worth remembering. There is no mark next to the definition yet, since the sequence is defined recursively and no values have yet been given for U2(1) and U2(2). Type in a value of 1 for both of these and then press the NUM button to switch to the NUM view. As you can see in the screenshot right, the NUM view shows the actual values in the sequence as a table. If you move the highlight into the U1 and U2 columns, you can press the button and see the sequence rule. You can experiment for yourself and see the result of pressing the button (see next page for an example). The button is also available as usual, but it is easier to use other methods. 100