/**
 * Double Exponential Moving Average (DEMA).
 * DEMA reduces lag by combining a single EMA and a double-smoothed EMA:
 * DEMA = 2 * EMA(source, period) - EMA(EMA(source, period), period).
 * Preserves NaN gaps and follows the same seeding semantics as `ema`.
 * @param source Input series
 * @param period Smoothing period (must be > 0)
 * @returns Float64Array of DEMA values (NaN where undefined)
 */
export declare function dema(source: ArrayLike<number>, period: number): Float64Array;