--- /dev/null
+'use strict';
+
+var log = Math.log, sqrt = Math.sqrt;
+
+module.exports = function (x) {
+ if (isNaN(x)) return NaN;
+ x = Number(x);
+ if (x < 1) return NaN;
+ if (x === 1) return 0;
+ if (x === Infinity) return x;
+ return log(x + sqrt(x * x - 1));
+};