If the rate of growth PP is proportional to itself, then with respect to time tt,
[dP]/dt=kPdPdt=kP, ....inverting both sides, .....dt/[dP]=[1]/[kPdtdP=1kP and so integrating both sides
intdt=int[dP]/[kP∫dt=∫dPkP, thus,..... t=1/klnP +t=1klnP+ a constant............[1][1]
Suppose PP is some value CC when t=0t=0, substituting
0=1/klnC+0=1klnC+ constant, therefore the constant = -1/klnC=−1klnC and so substituting this value for the constant in ...[1][1] we have ,
t= 1/k[ln P-lnC]t=1k[lnP−lnC] = 1/k ln[P/C]1kln[PC], therefore , kt=ln[p/C]kt=ln[pC][ theory of logs] and so
e^[kt]=P/Cekt=PC......giving P=Ce^[ktP=Cekt. The constant kk will represent the excess of births over deaths or vice versa for a decreasing rate.