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Search: id:A070897
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| A070897 |
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Number of ways of pairing numbers 1 to n with numbers n+1 to 2n such that each pair sums to a prime. |
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+0
6
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| 1, 1, 1, 1, 2, 4, 8, 36, 40, 49, 126, 121, 440, 2809, 11395, 32761, 132183, 881721, 3015500, 19642624, 106493895, 249987721, 1257922092, 4609187881, 29262161844, 189192811369, 1068996265025, 7388339422500, 67416357342087
(list; graph; listen)
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OFFSET
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1,5
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FORMULA
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a(n)=permanent(m), where the n-by-n matrix m is defined m(i,j) = 1 or 0, depending on whether i+j+n is prime or composite, respectively. - T. D. Noe (noe(AT)sspectra.com), Feb 10 2007
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EXAMPLE
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a(5)=2 because there are two ways: 1+10,2+9,3+8,4+7,6+5 and 1+6,2+9,3+10,4+7,5+8
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MATHEMATICA
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<< DiscreteMath`Combinatorica` listQpart2[ n_ ] := {n-#, #}&/@Range[ Floor[ (n-1)/2 ] ] Noe[ n_Integer ] := Module[ {it, permoid, po}, it=Union@Flatten[ Cases[ listQpart2[ # ], q_/; Max[ q ]<=2*n&&Max[ q ]>n ]& /@Select[ Range[ n+2, 3*n ], PrimeQ ], 1 ]; po=Position[ it, # ]&/@Range[ n ]; permoid=(Extract[ it, # ]-n)& /@(po /. {i_Integer, j_}->{i, 1} ); Length@Backtrack[ permoid, UnsameQ@@#&, Length[ # ]===n&, All ] ] Noe/@Range[ 2, 16 ] (from Wouter Meeussen)
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CROSSREFS
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Cf. A000341, A071058, A071059.
Sequence in context: A036544 A094334 A138744 this_sequence A018380 A058236 A102918
Adjacent sequences: A070894 A070895 A070896 this_sequence A070898 A070899 A070900
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KEYWORD
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nice,nonn
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AUTHOR
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T. D. Noe (noe(AT)sspectra.com), May 23 2002
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EXTENSIONS
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More terms from Don Reble (djr(AT)nk.ca), May 26 2002
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