secant {DFA} | R Documentation |

## secant method for detection of crossover points

### Description

Applies the secant method for detection of crossover points on the log-log curve.

### Usage

```
secant(x,y,npoint,size_fit)
```

### Arguments

`x` |
Vector of the decimal logarithm of the boxes sizes. |

`y` |
Vector of the decimal logarithm of the DFA calculated in each boxe. |

`npoint` |
Number of crossover points calculated on the log-log curve. |

`size_fit` |
Number of points of the two semi-curved fitted in the extremes of the log-log curve. |

### Value

`position` |
Position of the crossover point identified by the secant method. |

### Author(s)

Victor Barreto Mesquita

### Examples

```
# Example with the data referring to the log fluctuation
#channel curve data calculated for an epileptic subject
#extracted in the Physionet platform.
library(DFA)
data("EEGsignal")
x<-EEGsignal$`log10(boxes)`
y<-EEGsignal$`log10(DFA)`
plot(x,y,xlab="log10(boxes)",ylab="log10(DFA)")
secant(x,y,npoint=2,size_fit=8)
# Example with crossover point fixed in position=20.
library(DFA)
part1 <- seq(1,20)
part2 <- seq(20,1)
y = c(part1,part2)
x<-seq(1,40)
plot(x,y)
secant(x,y,npoint=1,size_fit=8)
```

[Package

*DFA*version 1.0.0 Index]