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 A138479 a(n) = smallest prime p such that 2n + p^2 is another prime, or 0 if no such prime exists. 11
 3, 3, 5, 3, 3, 5, 3, 5, 5, 3, 3, 7, 0, 3, 7, 3, 3, 5, 3, 7, 5, 3, 5, 5, 3, 3, 5, 0, 3, 7, 3, 3, 29, 0, 3, 5, 3, 5, 5, 3, 5, 5, 0, 3, 7, 3, 3, 19, 3, 3, 5, 3, 5, 7, 0, 5, 5, 0, 3, 11, 3, 5, 5, 3, 3, 5, 0, 11, 5, 3, 3, 7, 0, 3, 7, 0, 3, 5, 3, 11, 7, 3, 5, 5, 3, 3, 5, 0, 7, 7, 3, 3, 5, 3, 3, 7, 0, 11, 5, 0 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS For numbers n such that a(n) = 0 see A138685. LINKS Alois P. Heinz, Table of n, a(n) for n = 1..10000 Eric Weisstein's World of Mathematics, Near-Square Prime EXAMPLE 11=2+3^2 hence a(1)=3, 13=4+3^2 hence a(2)=3, 31=6+5^2 hence a(3)=5. MAPLE a:= proc(n) local p;       if irem(n, 3)=1 and not isprime(2*n+9) then 0     else p:=2;          do p:= nextprime(p);             if isprime(2*n+p^2) then return p fi          od       fi     end: seq(a(n), n=1..100);  # Alois P. Heinz, Jun 16 2014 MATHEMATICA a = {}; Do[ p = 0; While[ (! PrimeQ[ 2*n + Prime[ p + 1 ]2 ]) && (p < 1000), p++ ]; If[ p < 1000, AppendTo[ a, Prime[ p + 1 ] ], AppendTo[ a, 0 ] ], {n, 1, 150} ]; a (* Artur Jasinski, Mar 26 2008 *) a[n_]:=If[Mod[n, 3]!=1, (For[m=1, !PrimeQ[2n+Prime[m]^2], m++ ]; Prime[m]), If[ !PrimeQ[2n+9], 0, 3]]; Table[a[n], {n, 100}] - Farideh Firoozbakht, Mar 28 2008 CROSSREFS Cf. A002373, A020481, A049613, A059324 (?). Sequence in context: A204903 A054906 A269733 * A202106 A136019 A242017 Adjacent sequences:  A138476 A138477 A138478 * A138480 A138481 A138482 KEYWORD nonn AUTHOR Philippe LALLOUET (philip.lallouet(AT)orange.fr), Mar 20 2008 EXTENSIONS More terms from Artur Jasinski and Farideh Firoozbakht, Mar 26 2008 STATUS approved

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Last modified October 17 08:36 EDT 2019. Contains 328107 sequences. (Running on oeis4.)