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

1, 3, 7, 9, 21, 23, 27, 29, 41, 43, 47, 49, 61, 63, 67, 69, 81, 83, 87, 89, 101, 103, 107, 109, 121, 123, 127, 129, 141, 143, 147, 149, 161, 163, 167, 169, 181, 183, 187, 189, 201, 203, 207, 209, 221, 223, 227, 229, 241, 243, 247, 249, 261, 263, 267, 269, 281

Offset

1,2

Comments

From Jianing Song, Apr 27 2019: (Start)

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

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

First 20 terms are congruences of 3^k mod 100. - Dario Vuksan, Jan 09 2023

Links

Reinhard Zumkeller, Table of n, a(n) for n = 1..10000

Index entries for linear recurrences with constant coefficients, signature (1,0,0,1,-1).

Formula

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

The conjecture above is correct. - Jianing Song, Apr 27 2019

a(n) = 5n + O(1). - Charles R Greathouse IV, Jan 09 2023

Mathematica

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

Prog

(Haskell)
a045797 n = a045797_list !! (n-1)
a045797_list = filter (even . (`mod` 10) . (`div` 10)) a045572_list
-- Reinhard Zumkeller, Dec 10 2011
(PARI) is(n)=gcd(n, 10)==1 && n\10%2==0 \\ Charles R Greathouse IV, Sep 24 2015

Crossrefs

Complement of A045798 with respect to A045572.

Sequence in context: A306124 A096102 A316157 * A118555 A056652 A014959

Adjacent sequences: A045794 A045795 A045796 * A045798 A045799 A045800

Keyword

nonn,base,easy,nice

Author

J. H. Conway

Extensions

More terms from Erich Friedman

Offset changed by Reinhard Zumkeller, Dec 10 2011

Status

approved