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.
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
KEYWORD
sign,easy
AUTHOR
Wesley Ivan Hurt, Sep 22 2018
STATUS
approved