Level and index in the modular group
1984 ◽
Vol 99
(1-2)
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pp. 115-126
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Keyword(s):
SynopsisIt is shown that the index of a congruence subgroup of the modular group cannot be less than the level of the subgroup. This allows a number of existence theorems about non-congruence subgroups.The level of a subgroup of the modular group can be defined in terms of the action on Q ∪ {∞}. We define a similar action to get information on congruence subgroups. In fact, we get a more powerful result, but this appears to be the most useful version.
2009 ◽
Vol 39
(3)
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pp. 351-362
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1989 ◽
Vol 105
(2)
◽
pp. 241-248
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Keyword(s):
1991 ◽
Vol 1991
(414)
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pp. 113-116
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1965 ◽
Vol 16
(4)
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pp. 831-831
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2014 ◽
Vol 17
(1)
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pp. 206-208
Keyword(s):
1995 ◽
Vol 138
◽
pp. 179-197
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Keyword(s):
1979 ◽
Vol 11
(2)
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pp. 117-123
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1978 ◽
Vol 19
(2)
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pp. 173-197
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Keyword(s):