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 A057538 Birthday set of order 5: numbers congruent to +/-1 modulo 2, 3, 4 and 5. 11
 1, 11, 19, 29, 31, 41, 49, 59, 61, 71, 79, 89, 91, 101, 109, 119, 121, 131, 139, 149, 151, 161, 169, 179, 181, 191, 199, 209, 211, 221, 229, 239, 241, 251, 259, 269, 271, 281, 289, 299, 301, 311, 319, 329, 331, 341, 349, 359, 361, 371, 379, 389, 391, 401, 409 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Also numbers congruent to +/-1 or +/-11 modulo 30 and numbers k where (k^2-1)/120 is an integer; all but the first two prime legs of Pythagorean triangles which also have prime hypotenuses appear within in this sequence (A048161). - Henry Bottomley, Jan 31 2002 Numbers k such that k^2 mod 30 == 1. - Gary Detlefs, Apr 16 2012 Primes in this sequence given by A045468. - Ray Chandler, Jul 29 2019 LINKS Michael De Vlieger, Table of n, a(n) for n = 1..10000 A. Feist, On the Density of Birthday Sets, The Pentagon, 60 (No. 1, Fall 2000), 31-35. A. Feist, Maple source for birthday sets. [Broken link] Index entries for linear recurrences with constant coefficients, signature (1,0,0,1,-1). FORMULA A093722(n) = (a(n)^2-1)/120. G.f.: x * (1 + 10*x + 8*x^2 + 10*x^3 + x^4) / ((1 - x) * (1 - x^4)). a(-1 - n) = -a(n). - Michael Somos, Jan 21 2012 4*a(n) = 30*(n+1)-45 +5*(-1)^n+6*(-1)^floor((n+1)/2). - R. J. Mathar, Jul 30 2019 EXAMPLE 229 is congruent to 1 mod 2, 1 mod 3, 1 mod 4 and -1 mod 5. x+ 11*x^2 + 19*x^3 + 29*x^4 + 31*x^5 + 41*x^6 + 49*x^7 + 59*x^8 + 61*x^9 + ... MAPLE for n from 1 to 409 do if (n^2 mod 30 =1) then print(n) fi od; # Gary Detlefs, Apr 17 2012 MATHEMATICA a057538[n_] := Block[{f},   f[x_] :=    If[Mod[x, #] == 1 || Mod[x, #] == # - 1, True, False] & /@     Range[2, 5]; Select[Range[n], DeleteDuplicates[f[#]] == {True} &]]; a057538[409] (* Michael De Vlieger, Dec 26 2014 *) PROG (PARI) {a(n+1) = (n\4*3 + n%4)*10 + (-1)^(n\2)} /* Michael Somos, Oct 17 2006 */ CROSSREFS Cf. A007310, A045468, A057539, A057540, A057541, A093722. Sequence in context: A224380 A125771 A158290 * A123976 A045468 A196095 Adjacent sequences:  A057535 A057536 A057537 * A057539 A057540 A057541 KEYWORD easy,nonn AUTHOR Andrew R. Feist (andrewf(AT)math.duke.edu), Sep 06 2000 EXTENSIONS Corrected by Henry Bottomley, Jan 31 2002 Offset corrected to 1 by Ray Chandler, Jul 29 2019 STATUS approved

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Last modified August 19 08:26 EDT 2019. Contains 326115 sequences. (Running on oeis4.)