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 A319551 a(n) = 1*2*3*4*5*6*7*8*9*10 - 11*12*13*14*15*16*17*18*19*20 + 21*22*23*24*25*26*27*28*29*30 - ... + (up to n). 8
 1, 2, 6, 24, 120, 720, 5040, 40320, 362880, 3628800, 3628789, 3628668, 3627084, 3604776, 3268440, -2136960, -94389120, -1760693760, -33518499840, -670438944000, -670438943979, -670438943538, -670438933374, -670438688976, -670432568400, -670273178400 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS In general, for alternating sequences that multiply the first k natural numbers, and subtract/add the products of the next k natural numbers (preserving the order of operations up to n), we have a(n) = (-1)^floor(n/k) * Sum_{i=1..k-1} (1-sign((n-i) mod k)) * (Product_{j=1..i} (n-j+1)) + Sum_{i=1..n} (-1)^(floor(i/k)+1) * (1-sign(i mod k)) * (Product_{j=1..k} (i-j+1)). Here k=10. An alternating version of A319212. LINKS FORMULA a(n) = (-1)^floor(n/10) * Sum_{i=1..9} (1-sign((n-i) mod 10)) * (Product_{j=1..i} (n-j+1)) + Sum_{i=1..n} (-1)^(floor(i/10)+1) * (1-sign(i mod 10)) * (Product_{j=1..10} (i-j+1)). EXAMPLE a(1) = 1; a(2) = 1*2 = 2; a(3) = 1*2*3 = 6; a(4) = 1*2*3*4 = 24; a(5) = 1*2*3*4*5 = 120; a(6) = 1*2*3*4*5*6 = 720; a(7) = 1*2*3*4*5*6*7 = 5040; a(8) = 1*2*3*4*5*6*7*8 = 40320; a(9) = 1*2*3*4*5*6*7*8*9 = 362880; a(10) = 1*2*3*4*5*6*7*8*9*10 = 3628800; a(11) = 1*2*3*4*5*6*7*8*9*10 - 11 = 3628789; a(12) = 1*2*3*4*5*6*7*8*9*10 - 11*12 = 3628668; a(13) = 1*2*3*4*5*6*7*8*9*10 - 11*12*13 = 3627084; a(14) = 1*2*3*4*5*6*7*8*9*10 - 11*12*13*14 = 3604776; a(15) = 1*2*3*4*5*6*7*8*9*10 - 11*12*13*14*15 = 3268440; a(16) = 1*2*3*4*5*6*7*8*9*10 - 11*12*13*14*15*16 = -2136960; a(17) = 1*2*3*4*5*6*7*8*9*10 - 11*12*13*14*15*16*17 = -94389120; a(18) = 1*2*3*4*5*6*7*8*9*10 - 11*12*13*14*15*16*17*18 = -1760693760; a(19) = 1*2*3*4*5*6*7*8*9*10 - 11*12*13*14*15*16*17*18*19 = -33518499840; etc. MATHEMATICA a[n_]:=(-1)^Floor[n/10]*Sum[(1-Sign[Mod[n-i, 10]])*Product[n-j+1, {j, 1, i}], {i, 1, 9}]+Sum[(-1)^(Floor[i/10]+1)*(1-Sign[Mod[i, 10]])*Product[i-j+1, {j, 1, 10}], {i, 1, n}]; Array[a, 30] (* Stefano Spezia, Sep 23 2018 *) CROSSREFS For similar sequences, see: A001057 (k=1), A319373 (k=2), A319543 (k=3), A319544 (k=4), A319545 (k=5), A319546 (k=6), A319547 (k=7), A319549 (k=8), A319550 (k=9), this sequence (k=10). Cf. A319212. Sequence in context: A152702 A072133 A230232 * A232985 A319212 A276844 Adjacent sequences:  A319548 A319549 A319550 * A319552 A319553 A319554 KEYWORD sign,easy AUTHOR Wesley Ivan Hurt, Sep 22 2018 STATUS approved

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Last modified June 24 02:08 EDT 2021. Contains 345407 sequences. (Running on oeis4.)