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A255904
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Numbers that are neither triangular nor the sum of a positive triangular number and a prime.
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0
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OFFSET
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1,1
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COMMENTS
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Probably finite.
First four terms are primes, and are in A065397.
If it exists, a(6) > 5*10^6. - Derek Orr, Mar 12 2015
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LINKS
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EXAMPLE
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a(2) = 7 is in the sequence because 7 is not a triangular number, and cannot be written as a sum of the primes < 7 {2, 3, 5} and one of the positive triangular numbers < 7 {1, 3, 6}.
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PROG
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(Magma) lst:=[]; r:=21; for n in [0..r*(r+1)/2] do if not IsSquare(8*n+1) then f:=0; k:=1; while k*(k+1)/2 lt n-1 do if IsPrime(n-Truncate(k*(k+1)/2)) then f:=1; break; end if; k+:=1; end while; if IsZero(f) then Append(~lst, n); end if; end if; end for; lst;
(PARI) a(n)=k=1; while(n-(t=k*(k+1)/2)>=0, if(isprime(n-t)||n==t, return(k)); k++)
n=1; while(n<10^5, if(!a(n), print1(n, ", ")); n++) \\ Derek Orr, Mar 12 2015
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CROSSREFS
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KEYWORD
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nonn,fini
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AUTHOR
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STATUS
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approved
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