[R] How to find when a value is reached given a function?

[Luigi Marongiu]

Hello
I am trying to simulate a PCR by running a logistic equation. So I set
the function:
```
PCR <- function(initCopy, dupRate, Carry) {
  ROI_T = initCopy
  A = array()
  for (i in 1:45) {
    ROI_TplusOne <- ROI_T * dupRate * (1 - ROI_T/Carry)
    A[i] <- ROI_TplusOne
    ROI_T <- ROI_TplusOne
  }
  return(A)
}
```
Which returns an array that follows the logistic shape, for instance
```
d <- 2
K <- 10^13
A_0 <- 10000
PCR_array <- PCR(A_0, d, K)
plot(PCR_array)
```
Given the formula `ROI_TplusOne <- ROI_T * dupRate * (1 -
ROI_T/Carry)`, is it possible to determine at what time point `i` a
given threshold is reached? For instance, what fractional value of i
returns 1000 000 copies?
Thank you