According to the theory of relativity stabile acceleration will require more and more energy, or the speed will gradually drop
How can the influence of the speed decrease be calculated (which equation can I use)

According to the theory of relativity stabile acceleration will require more and more energy, or the speed will gradually drop
How can the influence of the speed decrease be calculated (which equation can I use)
The speed (velocity) won't drop of course, I assume you mean it is the rate of acceleration that will drop.
This is not my speciality but I think I recall the momentum instead of being p = mv becomes p=mv/sq.rt.(1v˛/c˛), so as you get closer to c, the denominator decreases and the momentum (and hence the energy required to accelerate the thing) grows at a faster than linear rate. So a linear rate of energy input has less and less effect on the speed, and ultimately you would need an infinite amount of it to get to c.
But I know there are far better qualified people on the forum to give you an answer.
Have a look at post #265 of this ongoing thread :
Can General Relativity model causation of time dilation?
It gives the relevant formulas there.
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