code: func (hc *HeaderChain) GetHeaderByNumber(number uint64) *types.Header {
  	hash := rawdb.ReadCanonicalHash(hc.chainDb, number)
  	if hash == (common.Hash{}) {
  		return nil
  	}
  	return hc.GetHeader(hash, number)
  }
docs: // GetHeaderByNumber retrieves a block header from the database by number,
  // caching it (associated with its hash) if found.