Typed states for the displayed outbound observations.
Source: paper_references, paper_reference_links, observed 2026-08-06T15:02:59.023436Z
Paper Citation Record · LEDGER
As of 13 August 2026, this Paper Citation Record lists 73 of 73 outbound references and 0 inbound Pith citation observations for arXiv:2507.17227.
A citation records a reference. It does not transfer a finding from one paper to another.
Typed states for the displayed outbound observations.
Source: paper_references, paper_reference_links, observed 2026-08-06T15:02:59.023436Z
One-hop event checks from named stored sources.
Source: scholarly_work_events, retraction_status_cache, observed 2026-08-13T06:32:02.005865+00:00
Pith citing papers itemized under the disclosed page cap.
Source: paper_references, paper_reference_links
A source-named dated measurement, never combined with another source.
Source: cited_works
73 of 73 outbound references displayed
External citation measurements
No source-named external measurement is stored.
Observation 13ab6fc3-1f3e-4e01-b9d3-9e7dd81ff218 · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Wang and S.-C
Reference 1
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation 22e7525f-b9a5-4e7b-a4ba-57ac96fb0924 · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Zhang, Topological states of quantum matter, Phys ics 1, 6 (2008)
Reference 2
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation 5178d5e9-5d36-4512-abde-103bda5d218f · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Sahin, H
Reference 3
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation 75b7d8ce-6b4f-457f-abf3-a9eb8222a2b3 · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Rafi-Ul-Islam, Z
Reference 4
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation 1bc98659-33e0-490f-8056-3c82499b936a · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Fujita, M
Reference 5
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation 0bc02966-6ef0-4394-baad-d70698f275f3 · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Rafi-Ul-Islam, Z
Reference 6
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation ed5e7755-b2cd-443f-84ea-4136c278acc1 · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Unresolved cited work
Reference 7
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation 1c65cec4-689d-4906-b1e6-ae325c2b907c · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Rafi-Ul-Islam, Z
Reference 8
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation 1ac109e5-a982-45c3-803c-000661ecf4bd · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Rafi-Ul-Islam, Z
Reference 9
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation f7a383a4-bdab-4832-b26a-dd1740b7253a · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Trebin, The topology of non-uniform media in cond ensed matter physics, Adv
Reference 10
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation 3312164f-d9eb-4279-b1ab-90bf3ed3fed2 · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Unresolved cited work
Reference 11
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation bf885c44-0c2f-48c1-8fa9-ef23cc1294bb · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Rafi-Ul-Islam, Z
Reference 12
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation 8c31a01c-82ec-473a-b09a-fea5cbd45b70 · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Laflorencie, Quantum entanglement in condensed matt er systems, Phys
Reference 13
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation b792b8f3-e038-4508-bf31-1bf2ce919b79 · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Unresolved cited work
Reference 14
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation 5921b635-725d-414e-b06b-a3e9e9f7a988 · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Unresolved cited work
Reference 15
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation 6dec192a-7868-4ff2-a12a-f607bb569d48 · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Gr¨ oning, S
Reference 16
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation 7aaad44a-37ba-4a54-aa56-7541151b3d86 · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Rafi-Ul-Islam, Z
Reference 17
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation 670dfc86-2f50-406b-8498-9ac5514096e3 · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Unresolved cited work
Reference 18
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation b4c1f539-bad3-4e29-915e-f8f8ba1c46b8 · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Unresolved cited work
Reference 19
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation 7bea252b-7496-473e-92da-29103c32f3cd · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Unresolved cited work
Reference 20
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation 2e64ae05-b86b-433a-a024-1dcee8d5955f · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Unresolved cited work
Reference 21
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation 9766200e-364b-4d45-b6d5-a6d1f29f15d8 · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Obana, F
Reference 22
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation fb0b3647-42e3-4cf9-9ab1-e40fbdff2792 · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Rafi-Ul-Islam, Z
Reference 23
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation 5deabcf1-fc5c-426c-aacf-a7053cfb1b9b · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Nakada, M
Reference 24
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation c6361c5f-da00-426c-95f6-836762f08a02 · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Hafezi, S
Reference 25
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation b3cb8031-6217-48ca-9228-976e2d51b7c4 · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Unresolved cited work
Reference 26
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation e0557dee-d3ab-4ec5-be89-1ffb5914757c · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Unresolved cited work
Reference 27
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation 3fe7cf38-ebe0-4ecf-a2b2-7ed7d025a517 · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Unresolved cited work
Reference 28
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation 58a5a3af-7ca4-4993-b52a-98dac19387be · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Unresolved cited work
Reference 29
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation d85f7e6e-666c-4b19-8b02-77cfa68fe08a · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Ashida, Z
Reference 30
Source-reported events for the cited work
Unavailable: canonical work link unavailable.
Observation 9cb583dd-06b7-4d85-b843-64844cc494ec · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Unresolved cited work
Reference 31
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation 5044b1c4-10c3-4b85-b5d5-cfaa401aa51a · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Rafi-Ul-Islam, Z
Reference 32
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation 8800432a-d91a-489b-a7c7-d268e077f3e0 · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Kawabata, K
Reference 33
Source-reported events for the cited work
Unavailable: canonical work link unavailable.
Observation 18c8ace9-a69e-4f9e-a5b2-5229e66e0b86 · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Rafi-Ul-Islam, Z
Reference 34
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation 8a37985f-cb3f-43d0-adb5-3fa3229c630b · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Unresolved cited work
Reference 35
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation da9d6cf6-7d14-4caf-beb4-fa38cdbc5bcb · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Unresolved cited work
Reference 36
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation 0a192712-4ed5-400f-bc8d-0d0cf97cd4d7 · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Koshino, T
Reference 37
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation 039105d0-a283-4e47-8878-085fbbd47fff · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Rafi-Ul-Islam, Z
Reference 38
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation 372101b0-aeef-45a9-8ddd-a9a6870835dd · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Unresolved cited work
Reference 39
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation 5412102b-1fc5-4b7c-b9d4-65fb5fd2934f · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Unresolved cited work
Reference 40
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation 0a0d90a0-b3f4-4599-aedf-3e76e172b5ce · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Imhof, C
Reference 41
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation 2aed3f74-0794-44a3-9f8b-bed26eb16567 · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Zhu and G
Reference 42
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation 048c6f27-4b12-46d4-8e03-434d19be9e2b · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Proctor, P
Reference 43
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation 1690df6e-f7cd-4321-bc35-b767790999ad · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Rafi-Ul-Islam, Z
Reference 44
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation dabcb8e3-7975-4096-a094-c1d3da086fd4 · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Unresolved cited work
Reference 45
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation c504d7ae-e2b7-4fa3-b3ec-babaae113c6f · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Fu, Z.-A
Reference 46
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation bc81d4e2-cd5a-40b8-a901-2b9f391f4491 · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Rafi-Ul-Islam, Z
Reference 47
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation 1e649649-d8a4-4bbf-a135-769923a71356 · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Helbig, T
Reference 48
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation 900e2190-dfc6-4f70-a6e2-a6a170dbe71d · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Zhang, B
Reference 49
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation 5489c269-cee2-4cc0-a0a3-e83fd7970b25 · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Unresolved cited work
Reference 50
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation f2f9f749-ac68-4cd8-90ca-f005557de6e8 · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Rafi-Ul-Islam, Z
Reference 51
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation cb969b73-69b9-46d4-8cae-7a3318abd7c4 · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Sahin, Z
Reference 52
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation cd112d47-a87b-4632-9a50-3b3aaebea10c · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Hofmann, T
Reference 53
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation 8a3e840b-3b1f-45fa-92a8-23bf51949889 · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Rafi-Ul-Islam, Z
Reference 54
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation a8471610-cec7-4922-9172-e74527c920f9 · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Unresolved cited work
Reference 55
Source-reported events for the cited work
Unavailable: canonical work link unavailable.
Observation 7b391704-0ecc-4ab5-91f1-a0b5d6edd666 · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Barik, A
Reference 56
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation 97b6e0da-b707-40e5-9bc5-ec00b3564e7d · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Soskin, S
Reference 57
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation 6f3f2dc8-1f40-4023-9f46-c6ec49d56ccd · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Qi and S.-C
Reference 58
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation 08ac4894-fcc2-4261-ac6c-f218aecea3a5 · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Zhang, Z
Reference 59
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation ac981e43-603e-4689-87e6-4e6b769dc822 · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Unresolved cited work
Reference 60
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation 37e9fe6c-7d14-438f-9fe6-cd9b0fc9eb27 · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Unresolved cited work
Reference 61
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation 8727ad2b-4183-4976-a0a2-b76cb7e959e2 · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Unresolved cited work
Reference 62
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation 6e60f64d-1a15-47ef-b25f-15589498ab79 · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Unresolved cited work
Reference 63
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation 301f2714-4c00-4756-b983-b03fd69265d2 · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Rafi-Ul-Islam, Z
Reference 64
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation d491ef8e-752e-44fb-8c50-f383004a5441 · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Rafi-Ul-Islam, H
Reference 65
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation b6376fc9-77ac-4413-925f-5ffefc6b2fbc · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Unresolved cited work
Reference 66
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation d5887cbd-bd1b-4a52-860d-6d4bfa6a1429 · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Unresolved cited work
Reference 67
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation 35c1bb77-999b-45c6-903f-cf142c79c876 · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Wang et al
Reference 68
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation 6b6904ac-6350-4dfc-98e6-f1c9b953ca48 · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Unresolved cited work
Reference 69
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation cc7086a5-f193-44f5-b965-c547e9a53362 · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Unresolved cited work
Reference 70
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation 30c6b9ba-c6c2-4c35-961a-5cd062f47be4 · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Unresolved cited work
Reference 71
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation 05c9073f-d63e-4f77-ad4c-d2586e4ed63c · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Unresolved cited work
Reference 72
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
Observation 9c043c09-a98f-4800-a2b7-6b9e026497c0 · outbound
Topological Zero Modes in Non-Hermitian Topolectrical Systems: Size and Impedance Control Unresolved cited work
Reference 73
Source-reported events for the cited work
No event found in the named queried sources as of 2026-08-13T06:32:02.005865+00:00.
No inbound Pith citation observations are available.