"Fσ" meaning in Translingual

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        "3": "fermé",
        "4": "",
        "5": "closed"
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      "expansion": "French fermé (“closed”)",
      "name": "der"
    }
  ],
  "etymology_text": "From French fermé (“closed”) for the 'F' and French somme (“sum”) for the 'σ'. The 'σ' can be interpreted to mean \"countable union.\"",
  "forms": [
    {
      "form": "F_σ",
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      "expansion": "F_σ",
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  ],
  "lang": "Translingual",
  "lang_code": "mul",
  "pos": "symbol",
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          "langcode": "mul",
          "name": "Mathematical analysis",
          "orig": "mul:Mathematical analysis",
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            "Mathematics",
            "Formal sciences",
            "Sciences",
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            "Fundamental"
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        },
        {
          "kind": "topical",
          "langcode": "mul",
          "name": "Measure theory",
          "orig": "mul:Measure theory",
          "parents": [
            "Mathematics",
            "Formal sciences",
            "Sciences",
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            "Fundamental"
          ],
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        },
        {
          "kind": "topical",
          "langcode": "mul",
          "name": "Topology",
          "orig": "mul:Topology",
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            "Formal sciences",
            "Sciences",
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          ],
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        }
      ],
      "glosses": [
        "A countable union of closed sets."
      ],
      "hypernyms": [
        {
          "word": "Borel set"
        }
      ],
      "id": "en-Fσ-mul-symbol-Xsv4uoi1",
      "links": [
        [
          "mathematical analysis",
          "mathematical analysis"
        ],
        [
          "topology",
          "topology"
        ],
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          "countable",
          "countable"
        ],
        [
          "union",
          "union"
        ],
        [
          "closed set",
          "closed set"
        ]
      ],
      "raw_glosses": [
        "(mathematical analysis, topology) A countable union of closed sets."
      ],
      "topics": [
        "mathematical-analysis",
        "mathematics",
        "sciences",
        "topology"
      ],
      "wikipedia": [
        "Fσ set"
      ]
    }
  ],
  "word": "Fσ"
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      "name": "der"
    }
  ],
  "etymology_text": "From French fermé (“closed”) for the 'F' and French somme (“sum”) for the 'σ'. The 'σ' can be interpreted to mean \"countable union.\"",
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    {
      "form": "F_σ",
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  "hypernyms": [
    {
      "word": "Borel set"
    }
  ],
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  "lang_code": "mul",
  "pos": "symbol",
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        "mul:Topology"
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        "A countable union of closed sets."
      ],
      "links": [
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          "mathematical analysis",
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        ],
        [
          "topology",
          "topology"
        ],
        [
          "countable",
          "countable"
        ],
        [
          "union",
          "union"
        ],
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          "closed set",
          "closed set"
        ]
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