/**
 * Keltner Channels.
 * Returns `[middle(EMA of typical price), upper, lower]` where upper/lower are `middle ± mult * ATR`.
 * Preserves NaNs when inputs are invalid.
 * @param high High price series
 * @param low Low price series
 * @param close Close price series
 * @param period Lookback period (must be > 0)
 * @param mult Multiplier applied to ATR
 * @returns Tuple `[middle, upper, lower]` as Float64Array
 */
export declare function keltner(high: ArrayLike<number>, low: ArrayLike<number>, close: ArrayLike<number>, period: number, mult: number): [Float64Array, Float64Array, Float64Array];