Both exponential growth and exponential decay follow an exponential curve. The rate of change is porportional to the value. Considering values at increments of the indpendent variable, each is equivalent to a geometric sequence.
If the base of the equivalent geometric sequence is less than 1, the relation is one of exponential decay. If the base is greater than 1, the relation is one of exponential growth.
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If the base of natural logarithms is used, the function is growth when the exponent is positive; decay when the exponent is negative.