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A045797 Evenish numbers (prime to 10 and 10's digit is even). 14

%I

%S 1,3,7,9,21,23,27,29,41,43,47,49,61,63,67,69,81,83,87,89,101,103,107,

%T 109,121,123,127,129,141,143,147,149,161,163,167,169,181,183,187,189,

%U 201,203,207,209,221,223,227,229,241,243,247,249,261,263,267,269,281

%N Evenish numbers (prime to 10 and 10's digit is even).

%C From _Jianing Song_, Apr 27 2019: (Start)

%C Numbers congruent to {1, 3, 7, 9} mod 20.

%C Numbers k such that Kronecker(-20,k) = A289741(k) = +1. (End)

%H Reinhard Zumkeller, <a href="/A045797/b045797.txt">Table of n, a(n) for n = 1..10000</a>

%H <a href="/index/Rec#order_05">Index entries for linear recurrences with constant coefficients</a>, signature (1, 0, 0, 1, -1).

%F Conjecture a(n) = a(n-1)+a(n-4)-a(n-5). G.f.: x*(1+2*x+4*x^2+2*x^3+11*x^4) / ((1-x)^2*(1+x)*(1+x^2)). - _Colin Barker_, Apr 14 2012

%F The conjecture above is correct. - _Jianing Song_, Apr 27 2019

%t Flatten[Table[10n+{1,3,7,9},{n,0,30,2}]] (* _Harvey P. Dale_, Dec 05 2012 *)

%o (Haskell)

%o a045797 n = a045797_list !! (n-1)

%o a045797_list = filter (even . (`mod` 10) . (`div` 10)) a045572_list

%o -- _Reinhard Zumkeller_, Dec 10 2011

%o (PARI) is(n)=gcd(n,10)==1 && n\10%2==0 \\ _Charles R Greathouse IV_, Sep 24 2015

%Y Complement of A045798 with respect to A045572.

%K nonn,base,easy,nice

%O 1,2

%A _J. H. Conway_

%E More terms from _Erich Friedman_.

%E Offset changed by _Reinhard Zumkeller_, Dec 10 2011

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Last modified February 21 19:49 EST 2020. Contains 332110 sequences. (Running on oeis4.)