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 A092487 a(n) = least k such that {n+1, n+2, n+3, ... n+k} has a subset the product of whose members with n is a square. 5
 0, 4, 5, 0, 5, 6, 7, 7, 0, 8, 11, 8, 13, 7, 9, 0, 17, 9, 19, 10, 7, 11, 23, 8, 0, 13, 8, 12, 29, 12, 31, 13, 11, 17, 13, 0, 37, 19, 13, 10, 41, 14, 43, 11, 15, 23, 47, 6, 0, 13, 17, 13, 53, 16, 11, 16, 19, 29, 59, 15, 61, 31, 14, 0, 13, 14, 67, 17 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS For n>1, n + a(n) is composite and n + a(n) is square if and only if n is square. - David A. Corneth, Oct 22 2016 REFERENCES R. K. Guy, Unsolved Problems in Number Theory, B30. LINKS Peter Kagey, Table of n, a(n) for n = 1..10000 FORMULA a(n) = A006255(n) - n. - Peter Kagey, Oct 22 2016 a(n^2) = 0, a(p) = p for prime p > 3. - David A. Corneth, Oct 22 2016 EXAMPLE a(48)=6 because 48*(48+2)*(48+6) is a square, but you can't square 48 with numbers from (48+1) to (48+5). MATHEMATICA Table[k = 0; Which[IntegerQ@ Sqrt@ n, k, And[PrimeQ@ n, n > 3], k = n, True, While[Length@ Select[n Map[Times @@ # &, n + Rest@ Subsets@ Range@ k], IntegerQ@ Sqrt@ # &] == 0, k++]]; k, {n, 40}] (* Michael De Vlieger, Oct 26 2016 *) PROG (PARI) a(n) = {if(issquare(n), return(0)); if(isprime(n), if(n>3, return(n), return(n+2) )); my(l = List([n, n+1]), m=2); while(1, for(i=1, #l-2, forvec(v = vector(i, j, [2, #l-1]), if(issquare(l * l[#l] * prod(k=1, #v, l[v[k]])), return(l[#l] - n)), 2)); listput(l, n+m); m++)} \\ David A. Corneth, Oct 22 2016 CROSSREFS Cf. A006255, A092488. Sequence in context: A186930 A159567 A164357 * A322505 A192041 A132022 Adjacent sequences:  A092484 A092485 A092486 * A092488 A092489 A092490 KEYWORD nonn,easy AUTHOR Don Reble, Apr 03 2004 STATUS approved

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Last modified January 24 07:18 EST 2020. Contains 331189 sequences. (Running on oeis4.)