Worked solution
Observe an alternating convergent recurrence
A sequence of real numbers satisfies for . Use a calculator to determine its behaviour when .
CJC · Discussion · Question 2
Answer
Neither increasing nor decreasing; converges numerically to (3 s.f.).
Generate the calculator sequence
Use sequence mode with and . The source calculator table is reproduced to three decimal places:
| n | xₙ (3 d.p.) |
|---|---|
| 1 | -2.000 |
| 2 | -0.135 |
| 3 | -0.873 |
| 4 | -0.418 |
| 5 | -0.659 |
| 6 | -0.518 |
| 7 | -0.596 |
| 8 | -0.551 |
| 9 | -0.576 |
| 10 | -0.562 |
| 11 | -0.570 |
| 12 | -0.565 |
| 13 | -0.568 |
| 14 | -0.567 |
| 15 | -0.567 |
| 16 | -0.567 |
| 17 | -0.567 |
| 18 | -0.567 |
| 19 | -0.567 |
| 20 | -0.567 |
| 21 | -0.567 |
How did this part go?
Describe the behaviour
The terms alternate on either side of a limiting value, so the full sequence is neither increasing nor decreasing. The calculator values approach (3 significant figures). A fixed point satisfies ; numerical solution gives . The table is numerical evidence of this behaviour, not by itself a proof of convergence.
How did this part go?
Common mistakes
Do not mistake alternating sides of a limit for divergence.