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 A051728 Smallest number at distance 2n from nearest prime. 21
 2, 0, 23, 53, 409, 293, 211, 1341, 1343, 2179, 3967, 15705, 16033, 19635, 31425, 24281, 31429, 31431, 31433, 155959, 38501, 58831, 203713, 268343, 206699, 370311, 370313, 370315, 370317, 1349591, 1357261, 1272749, 1357265, 1357267 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,1 COMMENTS a(0) = 2. For n > 0, let f(m) = minimal distance from m to closest prime (excluding m itself). The a(n) = min { m : f(m) = 2n }. f(m) is tabulated in A051700. - R. J. Mathar, Nov 18 2007 LINKS MAPLE A051700 := proc(m) if m <= 2 then op(m+1, [2, 1, 1]) ; else min(nextprime(m)-m, m-prevprime(m)) ; fi ; end: A051728 := proc(n) local m ; if n = 0 then RETURN(2); else for m from 0 do if A051700(m) = 2 * n then RETURN(m) ; fi ; od: fi ; end: seq(A051728(n), n=0..20) ; # R. J. Mathar, Nov 18 2007 MATHEMATICA a[n_] := Module[{m}, If[n == 0, Return[2], For[m = 0, True, m++, If[Min[NextPrime[m]-m, m-NextPrime[m, -1]] == 2*n, Return[m]]]]]; Table[Print[an = a[n]]; an, {n, 0, 33}] (* Jean-François Alcover, Feb 11 2014, after R. J. Mathar *) Join[{2}, With[{t=Table[{n, Min[n-NextPrime[n, -1], NextPrime[n]-n]}, {n, 0, 1358000}]}, Table[SelectFirst[t, #[[2]]==2k&], {k, 33}]][[All, 1]]] (* Harvey P. Dale, Aug 13 2019 *) CROSSREFS Related sequences: A023186-A023188, A046929-A046931, A051650, A051652, A051697-A051702, A051728-A051730. Cf. A132470. Sequence in context: A106708 A138551 A133490 * A201954 A005359 A008842 Adjacent sequences:  A051725 A051726 A051727 * A051729 A051730 A051731 KEYWORD nonn,easy,nice AUTHOR EXTENSIONS More terms from James A. Sellers, Dec 07 1999 STATUS approved

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Last modified October 17 01:03 EDT 2019. Contains 328103 sequences. (Running on oeis4.)