1,2

Contains the repunits R_n, (A000042 or A002275): For any m in the sequence (divisible by neither 2 nor 5), Euler's theorem (i.e. m|10^m-1=9*R_n) guarantees that R_n is always some multiple of m (see A099679) and thus forms a subsequence. - Lekraj Beedassy (blekraj(AT)yahoo.com), Oct 26 2004

Inverse formula: n = 4*floor(a(n)/10) + floor([a(n) mod 10]/3) + 1 - Carl R. White (oeisfan(AT)phodd.net), Feb 06 2008

Numbers ending with 1, 3, 7 or 9. [From Lekraj Beedassy (blekraj(AT)yahoo.com), Apr 04 2009]

A000035(a(n))*(1-A079998(a(n)))=1; complement of A065502. [From Reinhard Zumkeller (reinhard.zumkeller(AT)gmail.com), Nov 15 2009]

a(n)=-1+Sum_{k=0..n}{(1/12)*{5*(k mod 4)+11*[(k+1) mod 4]-[(k+2) mod 4]+5*[(k+3) mod 4]}} - Paolo P. Lava (ppl(AT)spl.at), Dec 20 2007

a(n) = 10*floor((n-1)/4) + 2*floor( (4*[(n-1) mod 4]+1)/3 ) + 1 a(n) = a(n-1) + 2 + 2*floor([(x+6) mod 10]/9) - Carl R. White (oeisfan(AT)phodd.net), Feb 06 2008

a(n) = 2*n + 2*floor((n-3)/4) + 1 - Kenneth Hammond (weregoose(AT)gmail.com), Mar 07 2008

a(n) = -1 + 2*n + 2*floor((n+1)/4) - Kenneth Hammond (weregoose(AT)gmail.com), Mar 25 2008

a(n)= a(n-1) +a(n-4) -a(n-5). G.f.: x*(1+2*x+4*x^2+2*x^3+x^4)/ ((1+x) * (x^2+1) * (x-1)^2). [From R. J. Mathar (mathar(AT)strw.leidenuniv.nl), Sep 22 2009]

a(18) = 10*floor(17/4) + 2*floor( (4*[17 mod 4]+1)/3 ) + 1

= 10*4 + 2*floor( (4*[1]+1)/3 ) + 1

= 40 + 2*floor(5/3) + 1

= 40 + 2*1 + 1

= 43

/*GNU bc code*/ scale=0; for(n=1; n<=100; n++) 10*((n-1)/4)+2*((4*((n-1)%4)+1)/3)+1 - Carl R. White (oeisfan(AT)phodd.net), Feb 06 2008

(Other) SAGE :[i for i in xrange(0, 156) if gcd(20, i) == 1]# [From Zerinvary Lajos (zerinvarylajos(AT)yahoo.com), May 18 2009]

See also A085820.

Sequence in context: A165631 A005818 A085820 this_sequence A069254 A105585 A080903

Adjacent sequences: A045569 A045570 A045571 this_sequence A045573 A045574 A045575

easy,nonn,new

Jeff Burch (gburch(AT)erols.com)