1,2

Also numbers n such that the sum of the 4-th powers of the first n positive integers is divisible by n, or A000538(n) = n*(n+1)(2*n+1)(3*n^2+3*n-1)/30 is divisible by n. - Alexander Adamchuk (alex(AT)kolmogorov.com), Jan 04 2007

A141256(a(n)) = n+1. - Reinhard Zumkeller (reinhard.zumkeller(AT)gmail.com), Jun 17 2008

Also the 7-rough numbers: positive integers that have no prime factors less than 7 [From Michael Porter (michael_b_porter(AT)yahoo.com), Oct 09 2009]

N. J. A. Sloane, Table of n, a(n) for n = 1..8000

Theodore E. Hahn, Title?

Theodore E. Hahn, Title?

Theodore E. Hahn, Title?

Eric Weisstein's World of Mathematics, Rough Number From MathWorld--A Wolfram Web Resource. [From Michael Porter (michael_b_porter(AT)yahoo.com), Oct 09 2009]

Index entries for sequences related to smooth numbers [From Michael Porter (michael_b_porter(AT)yahoo.com), Oct 09 2009]

a(n+8)=a(n)+30. a(n)=a(n-1)+a(n-8)-a(n-9). G.f.: x*(1+6*x+4*x^2+2*x^3+4*x^4+2*x^5+4*x^6+6*x^7+x^8)/((1+x)*(x^2+1)*(x^4+1)*( x-1)^2). [From R. J. Mathar (mathar(AT)strw.leidenuniv.nl), Feb 27 2009]

for i from 1 to 500 do if gcd(i, 30) = 1 then print(i); fi; od;

Select[ Range[ 300 ], GCD[ #1, 30 ]==1& ]

(PARI) isA007775(n) = gcd(n, 30)==1 [From Michael Porter (michael_b_porter(AT)yahoo.com), Oct 09 2009]

Cf. A000538, A054403.

Cf. A008364, A008365, A008366. [From R. J. Mathar (mathar(AT)strw.leidenuniv.nl), Feb 27 2009]

For k-rough numbers with other values of k, see A000027 A005408 A007310 A007775 A008364 A008365 A008366 A166061 A166063 [From Michael Porter (michael_b_porter(AT)yahoo.com), Oct 09 2009]

Sequence in context: A128974 A005776 A161850 this_sequence A070884 A135777 A090459

Adjacent sequences: A007772 A007773 A007774 this_sequence A007776 A007777 A007778

nonn,easy

N. J. A. Sloane (njas(AT)research.att.com).