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# Kodaira symbols

Kodaira symbols encode the type of reduction of an elliptic curve at a (finite) place.

The standard notation for Kodaira Symbols is as a string which is one of \(\rm{I}_m\), \(\rm{II}\), \(\rm{III}\), \(\rm{IV}\), \(\rm{I}^*_m\), \(\rm{II}^*\), \(\rm{III}^*\), \(\rm{IV}^*\), where \(m\) denotes a non-negative integer. These have been encoded by single integers by different people. For convenience we give here the conversion table between strings, the eclib coding and the PARI encoding.

Kodaira Symbol | Eclib coding | PARI Coding
--- | --- | ---
\(\rm{I}_0\) | \(0\) | \(1\)
\(\rm{I}^*_0\) | \(1\) | \(-1\)
\(\rm{I}_m\) \((m>0)\) | \(10m\) | \(m+4\)
\(\rm{I}^*_m\) \((m>0)\) | \(10m+1\) | \(-(m+4)\)
\(\rm{II}\) | \(2\) | \(2\)
\(\rm{III}\) | \(3\) | \(3\)
\(\rm{IV}\) | \(4\) | \(4\)
\(\rm{II}^*\) | \(7\) | \(-2\)
\(\rm{III}^*\) | \(6\) | \(-3\)
\(\rm{IV}^*\) | \(5\) | \(-4\)

AUTHORS:

- David Roe <roed@math.harvard.edu>
- John Cremona

`sage.schemes.elliptic_curves.kodaira_symbol.``KodairaSymbol`(symbol)

  Returns the specified Kodaira symbol.

  INPUT:

  - `symbol` (string or integer) – Either a string of the form “I0”, “I1”, ..., “In”, “II”, “III”, “IV”, “I0*”, “I1*”, ..., “In*”, “II*”, “III*”, or “IV*”, or an integer encoding a Kodaira symbol using PARI’s conventions.

  OUTPUT:

  (KodairaSymbol) The corresponding Kodaira symbol.

  EXAMPLES:

      sage: KS = KodairaSymbol
      sage: [KS(n) for n in range(1,10)]
      [I0, II, III, IV, I1, I2, I3, I4, I5]
      sage: [KS(-n) for n in range(1,10)]
      [I0*, II*, III*, IV*, I1*, I2*, I3*, I4*, I5*]
      sage: all([KS(str(KS(n)))==KS(n) for n in range(-10,10) if n!=0])
      True

class `sage.schemes.elliptic_curves.kodaira_symbol.``KodairaSymbol_class`(symbol)

  Bases: `sage.structure.sage_object.SageObject`

  Class to hold a Kodaira symbol of an elliptic curve over a \(p\)-adic local field.

  Users should use the `KodairaSymbol()` function to construct Kodaira Symbols rather than use the class constructor directly.

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