Final answer:
In the Keynesian model, a decline in investment demand will not cause the IS curve to shift down and to the right in the short run, but it will cause the IS curve to shift in the long run.
Step-by-step explanation:
In the Keynesian model, price stickiness is a key assumption that affects the relationship between investment demand and the IS-LM curves. A decline in investment demand will not cause the IS curve to shift down and to the left in the short run, but it will cause the IS curve to shift in the long run.
This is because price stickiness means that changes in investment demand do not immediately cause an equivalent change in output (GDP). Instead, in the short run, the decline in investment demand will lead to a decrease in output and a shift in the IS curve. But over time, as prices gradually adjust, the IS curve will shift back to its original position, as the economy returns to full employment.