Is there anybody who can tell me if there is an mathematical relationship between the flow of my buffer (in ml/min) and the retentiontime of my samplecompound

Yes, there is. For gradients, you need to use more a little more care to retain selectivity. You actually need to think in terms of elution volume rather than retention time. When you change your flow rate, your retention times change, however the elution volume for each peak remains constant.

I hope this isn't an answer to a homework question...

Fogot to add - it's a linear relationship...