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 A256245 a(n) is the smallest positive number m such that n+3*m is a square, or 0 if no such m exists. 0
 1, 0, 2, 4, 0, 1, 3, 0, 9, 2, 0, 8, 1, 0, 7, 3, 0, 6, 2, 0, 5, 1, 0, 4, 8, 0, 3, 7, 0, 2, 6, 0, 1, 5, 0, 15, 4, 0, 14, 3, 0, 13, 2, 0, 12, 1, 0, 11, 5, 0, 10, 4, 0, 9, 3, 0, 8, 2, 0, 7, 1, 0, 6, 12, 0, 5, 11, 0, 4, 10, 0, 3, 9, 0, 2, 8, 0, 1, 7, 0, 21, 6, 0, 20, 5, 0, 19, 4, 0, 18, 3, 0, 17, 2 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 LINKS FORMULA a(n)=0 iff n==2 mod 3 because 2 is quadratic nonresidue of 3. EXAMPLE 1 + 3*1 = 4 = 2^2, 3 + 3*2 = 9 = 3^2, 4 + 3*4 = 16 = 4^2. MATHEMATICA Table[m = 1; If[Mod[n, 3] == 2, m = 0, While[! IntegerQ[Sqrt[n + 3*m]], m++]]; m, {n, 100}] (* Michael De Vlieger, Mar 20 2015 *) PROG (PARI) a(n)=if(n==Mod(2, 3), return(0)); m=1; while(!issquare(n+3*m), m++); m vector(100, n, a(n)) \\ Derek Orr, Mar 22 2015 CROSSREFS Cf. A256243. Sequence in context: A130659 A083741 A258761 * A173004 A118343 A309148 Adjacent sequences:  A256242 A256243 A256244 * A256246 A256247 A256248 KEYWORD nonn,easy AUTHOR Zak Seidov, Mar 20 2015 STATUS approved

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Last modified January 21 04:24 EST 2021. Contains 340332 sequences. (Running on oeis4.)