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A144291
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Triangular numbers n*(n-1)/2 with n and n -1 nonprime.
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5
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0, 36, 45, 105, 120, 210, 231, 300, 325, 351, 378, 528, 561, 595, 630, 741, 780, 990, 1035, 1176, 1225, 1275, 1326, 1485, 1540, 1596, 1653, 1953, 2016, 2080, 2145, 2346, 2415, 2775, 2850, 2926, 3003, 3240, 3321, 3570, 3655, 3741, 3828, 4095, 4186, 4278
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OFFSET
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1,2
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LINKS
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FORMULA
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EXAMPLE
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If n=1, then 1*(1-1)/2=0=a(1).
If n=9, then 9*(9-1)/2=36=a(2).
etc.
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MAPLE
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p:= -1: Res:= NULL: count:= 0:
while count < 100 do
q:= p; p:= nextprime(p);
if p - q > 2 then
count:= count + p-q-2;
Res:= Res, seq(k*(k+1)/2, k=q+1..p-2);
fi
od:
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MATHEMATICA
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Reap[For[n = 1, n <= 100, n++, If[!PrimeQ[n] && !PrimeQ[n-1], Sow[n(n-1)/2] ] ] ][[2, 1]] (* Jean-François Alcover, Mar 27 2019 *)
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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