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A214036
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Numbers n such that floor(sqrt(1)) + floor(sqrt(2)) + floor(sqrt(3)) + ... + floor(sqrt(n)) is prime.
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1
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2, 3, 4, 5, 7, 8, 10, 14, 36, 37, 39, 42, 43, 44, 46, 47
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OFFSET
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1,1
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COMMENTS
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The sequence is complete. Indeed, let s(n) be the sum of floor(sqrt(k)) for k from 1 to n. It is easy to verify that s(n^2+j), for 0 <= j < (n+1)^2-n^2, is equal to n(j+1) + n(4n+1)(n-1)/6, which is always divisible by n or by n/6 for n > 6. - Giovanni Resta, Mar 26 2014
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LINKS
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EXAMPLE
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2 is a term because floor(sqrt(1))+floor(sqrt(2)) = 1+1 = 2 is prime;
14 is a term because floor(sqrt(1))+ ... +floor(sqrt(14)) = 1+1+1+2+2+2+2+2+3+3+3+3+3+3 = 31 is prime.
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MAPLE
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for n from 1 to q do a:=a+floor(sqrt(n)); if isprime(a) then print(n); fi; od; end:
Alternative program:
A214036_bis:=proc(q) local a, j, n; a:=0;
for n from 1 to q do for j from 1 to 2*n+1 do
a:=a+n; if isprime(a) then print(n^2+j-1); fi;
od; od; end:
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MATHEMATICA
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Position[Accumulate[Table[Floor[Sqrt[n]], {n, 50}]], _?PrimeQ]//Flatten (* Harvey P. Dale, Apr 14 2017 *)
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PROG
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(PARI)
default(realprecision, 66);
sm = 0; /* sum(n>=1, floor(sqrt(n)) */
for (n=1, 10^9, sm+=sqrtint(n); if (isprime(sm), print1(n, ", ")));
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CROSSREFS
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KEYWORD
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nonn,fini,full
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AUTHOR
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STATUS
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approved
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