# 逆正弦(近似) Inverse Sine Approximation
# 引数 値 ($x)
# 戻り値 逆正弦(ラジアン) ($InverseSineApproximation)
sub INVERSESINE{
	my ($x) = @_;
	my $InverseSineApproximation = 0;
	my $Sum = 0;
	my $X = abs($x);
	my $Num = 0;
	my $Den1 = 0;
	my $Den2 = 1;
	my $Den3 = 0;
	my $NumFactorial = 1;
	my $DenFactorial = 1;
	my $Pi = atan2(1, 1) * 4;
	my $Pi2 = $Pi / 2;
	my $Expansion = 20;

	# 値の確認
	if($X > $Pi2){
		return 0;
	}

	for(my $i = 0; $i <= $Expansion; $i++){
		# 分子
		$Num = $X ** ((2 * $i) + 1);
		# 分母
		$Den1 = 2 ** (2 * $i);
		$Den3 = (2 * $i) + 1;

		if($i != 0){
			# 分子 階乗
			for(my $j = (2 * $i); $j > ((2 * $i) - 1); $j--){
				$NumFactorial = $NumFactorial * $j;
			}

			# 分母 階乗
			for(my $j = $i; $j > ($i - 1); $j--){
				$DenFactorial = $DenFactorial * $j;
			}
			# 分母
			$Den2 = $DenFactorial * $DenFactorial;
		}
	
		# 逆正弦(近似) InverseSine 
		$Sum += ($NumFactorial * $Num) / ($Den1 * $Den2 * $Den3);
	}

	# 符号を負
	$Sum = -$Sum if($x < 0);

	# 逆正弦(近似) Inverse Sine 
	$InverseSineApproximation = $Sum;
	
	return $InverseSineApproximation;
}