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Comparison of H I 21cm absorption with corresponding molecular (rotational) absorption spectra allows us to constrain variation in y=alpha^2*g_p where alpha is the fine structure constant and g_p is the proton g-factor. We analyse spectra of two QSOs, PKS 1413+135 and TXS 0218+357, and derive values of dy/y at absorption redshifts of z=0.2467 and 0.6847 by simultaneous fitting of the H I 21cm and molecular lines. We find dy/y=(-0.20 +/- 0.44)*10^{-5} and dy/y=(-0.16 +/- 0."},"verification_status":{"content_addressed":true,"pith_receipt":true,"author_attested":false,"weak_author_claims":0,"strong_author_claims":0,"externally_anchored":false,"storage_verified":false,"citation_signatures":0,"replication_records":0,"graph_snapshot":true,"references_resolved":false,"formal_links_present":false},"canonical_record":{"source":{"id":"astro-ph/0101519","kind":"arxiv","version":2},"metadata":{"license":"","primary_cat":"astro-ph","submitted_at":"2001-01-29T22:49:39Z","cross_cats_sorted":[],"title_canon_sha256":"eff921d24d47c07f2958a0f0bda0bb3c93645ea0273c3f3ffcbac141b19c97f0","abstract_canon_sha256":"d3587ad0183156b1572aff60bc00e50bab63324aa025e4a4a2a4b2dd6f7aa902"},"schema_version":"1.0"},"receipt":{"kind":"pith_receipt","key_id":"pith-v1-2026-05","algorithm":"ed25519","signed_at":"2026-07-04T15:48:02.195016Z","signature_b64":"87060Idy1iTvMaMXh8o/agPMLDRGVI0yDaSofmusTVes2/B1UpOSGr3sPsgiVar5yh3ZIlJAn6zJHfyhAc5jAQ==","signed_message":"canonical_sha256_bytes","builder_version":"pith-number-builder-2026-05-17-v1","receipt_version":"0.3","canonical_sha256":"4a70d11db0af607cf77220741b392dc9fb435c0e22e502a394a19ad175b9b079","last_reissued_at":"2026-07-04T15:48:02.194648Z","signature_status":"signed_v1","first_computed_at":"2026-07-04T15:48:02.194648Z","public_key_fingerprint":"8d4b5ee74e4693bcd1df2446408b0d54"},"graph_snapshot":{"paper":{"title":"Improved constraints on possible variation of physical constants from H I 21cm and molecular QSO absorption lines","license":"","headline":"","cross_cats":[],"primary_cat":"astro-ph","authors_text":"F. 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