import type { CorrelationInput, CorrelationResult } from './types.js';
/**
 * Pearson correlation coefficient between two equal-length samples.
 *
 * @param input - Paired samples x and y
 * @returns Correlation coefficient r, r-squared, and sample size n
 * @throws RangeError if x and y lengths differ or contain fewer than 2 points
 * @remarks For valid input where one variable has zero variance (constant data),
 * the Pearson denominator is 0 and correlation is undefined; this returns
 * `{ r: 0, r2: 0, n }` as a finite sentinel rather than throwing or emitting NaN
 * (same valid-but-degenerate policy as the capability-index family — see ERRORS.md).
 */
export declare function correlation(input: CorrelationInput): CorrelationResult;
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