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A342376
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Number of non-commutative rings without 1 containing n elements.
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4
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0, 0, 0, 2, 0, 0, 0, 17, 2, 0, 0, 4, 0, 0, 0, 215, 0, 4, 0, 4, 0, 0, 0, 35, 2, 0, 23, 4, 0, 0, 0
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OFFSET
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1,4
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COMMENTS
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A ring without 1 is still a ring, although sometimes called a rng, or a non-unital ring, or a pseudo-ring (see Wikipedia links).
These are rings in which multiplication has no unit, and where there is at least one pair of non-commuting elements.
a(n)=0 if and only if n is squarefree.
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LINKS
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FORMULA
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EXAMPLE
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For n=4, there are 11 rings of order 4, 2 of which are without 1 and non-commutative, so a(4)= 2. Note that for these 2 rings, the abelian group under addition is the commutative Klein group Z/2Z + Z/2Z. These two rings are the last two rings described in the link Greg Dresden in reference: Ring 22.NC.1 and Ring 22.NC.2.
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CROSSREFS
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Number of non-commutative rings: A127708 (with 1 containing n elements), this sequence (without 1 containing n elements), A209401 (with n elements).
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KEYWORD
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nonn,more
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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