Calculating Fibonacci Sequence to n values or calculating a specific nth value

Fibonacci-Sequence

#Show all values in sequence up to number $seqnum
[int]$seqnum = 10
$fibseq = @()
for($i = 0; $i -lt $seqnum; $i++) {
    if ($i -eq 0 ) {
        $fibseq = $fibseq + 0
    }
    if ($i -eq 1 ) {
        $fibseq = $fibseq + $i
    }
    if ($i -gt 1 ) {
        $fibseq = $fibseq + ($fibseq[$i-2] + $fibseq[$i-1])
    }
}
$fibseq -join ','

#Return only value of the $seqnum number in the sequence
[int]$seqnum = 10
[double]$Phi = 1.618033988749895
$fibseq = 0,1,1,2
if ( $seqnum -le 3) {
        if ($seqnum -eq 1 ) {
            $fibval = $fibseq[$seqnum-1]
        }
        if ($seqnum -eq 2 ) {
            $fibval = $fibseq[$seqnum-1]
        }
        if ($seqnum -eq 3 ) {
            $fibval = $fibseq[$seqnum-1]
        }
} else {
    $fibval = [Math]::Round([Math]::Pow($Phi,$seqnum) / ($Phi + 2))
}
$fibval