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A007775 Numbers not divisible by 2, 3 or 5. 46
1, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 49, 53, 59, 61, 67, 71, 73, 77, 79, 83, 89, 91, 97, 101, 103, 107, 109, 113, 119, 121, 127, 131, 133, 137, 139, 143, 149, 151, 157, 161, 163, 167, 169, 173, 179, 181, 187, 191, 193, 197, 199, 203, 209 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Also numbers n such that the sum of the 4th 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, Jan 04 2007

A141256(a(n)) = n+1. - Reinhard Zumkeller, Jun 17 2008

Also the 7-rough numbers: positive integers that have no prime factors less than 7. - Michael B. Porter, Oct 09 2009

a(n) mod 3 has period 8, repeating [1,1,2,1,2,1,2,2]= (n mod 2) +floor(((n-1) mod 8)/7) - floor(((n-2) mod 8)/7)+1. floor(a(n)/3) is the set of numbers k such that k is congruent to {0,2,3,4,5,6,7,9} mod 10 = floor((5*n-2)/4)-floor((n mod 8)/6). - Gary Detlefs, Jan 08 2012

Numbers k such that C(k+3,3)==1 (mod k) and C(k+5,5)==1 (mod k). - Gary Detlefs, Sep 15 2013

a(n) mod 30 has period 8 repeating [1, 7, 11, 13, 17, 19, 23, 29]. The mean of these 8 numbers is 120/8 = 15. a(n)-15 mod 30 has period 8 repeating [-14, -8, -4, -2, 2, 4, 8, 14]. One half of the absolute value produces the symmetric sequence [7, 4, 2, 1, 1, 2, 4, 7] = A061501((n-1)+16 mod 8). - Gary Detlefs, Sep 24 2013

a(n) are exactly those positive integers m such that the sequence b(n) = n*(n + m)*(n + 2*m)*(n + 3*m)(n + 4*m)/120 is integral. Cf. A007310. - Peter Bala, Nov 13 2015

LINKS

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

P. Bala, A note on A007775

Eric Weisstein's World of Mathematics, Rough Number

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

Index entries for sequences related to smooth numbers

FORMULA

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). [R. J. Mathar, Feb 27 2009]

a(n) = 4*n-3-2*floor[(n-1)/8]+{1+(-1)^floor[(n-2)/2]}*(-1)^floor[(n-2)/4], n>=1. - Timothy Hopper, Mar 14 2010

a(1 - n) = -a(n). - Michael Somos, Feb 05 2011

Numbers k such that ((k^2 mod 48=1) or (k^2 mod 48=25)) and ((k^2 mod 120=1) or (k^2 mod 120=49)).[Gary Detlefs, Dec 30 2011]

Numbers k such that k^2 mod 30 is 1 or 19. - Gary Detlefs, Dec 31 2011

a(n) = 3*(floor((5*n-2)/4)-floor((n mod 8)/6))+ (n mod 2) +floor(((n-1) mod 8)/7) - floor(((n-2) mod 8)/7)+1. - Gary Detlefs, Jan 08 2012

a(n) = 4*n-3 + 2*(floor((n+6)/8) - floor((n+4)/8) - floor((n+2)/8) + floor(n/8) - floor((n-1)/8)), n>=1. From the o.g.f. given above by R. J. Mathar (with the denominator written as (1-x^8)*(1-x)), and a two step reduction of the floor functions. Compare with Hopper's and Detlefs' formulas above. - Wolfdieter Lang, Jan 26 2012

a(n) = (6*f(n)-3 +(-1)^f(n))/2, where f(n)= n + floor(n/4)+ floor(((n+4) mod 8)/6). - Gary Detlefs, Sep 15 2013

a(n) = 30*floor((n-1)/8) + 15 + 2*f((n-1) mod 8 + 16)*(-1)^floor(((n+3) mod 8)/4), where f(n)= (n*(n+1)/2+1) mod 10. - Gary Detlefs, Sep 24 2013

a(n) = 3*n + 6*floor(n/8) + (n mod 2) - 2*floor(((n-2) mod 8)/6) - 2*floor(((n-2) mod 8)/7) + 1. - Gary Detlefs, Jun 01 2014

MAPLE

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

for k from 1 to 300 do if ((k^2 mod 48=1) or (k^2 mod 48=25)) and ((k^2 mod 120=1) or (k^2 mod 120=49)) then print(k) fi od. # Gary Detlefs, Dec 30 2011]

MATHEMATICA

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

Select[Range[250], Mod[#, 2]>0&&Mod[#, 3]>0&&Mod[#, 5]>0&] (* Vincenzo Librandi, Feb 08 2014 *)

a[ n_] := Quotient[ n, 8, 1] 30 + {1, 7, 11, 13, 17, 19, 23, 29}[[Mod[n, 8, 1]]]; (* Michael Somos, Jun 02 2014 *)

PROG

(PARI) isA007775(n) = gcd(n, 30)==1 \\ Michael B. Porter, Oct 09 2009

(PARI) {a(n) = n\8 * 30 + [ -1, 1, 7, 11, 13, 17, 19, 23][n%8 + 1]} /* Michael Somos, Feb 05 2011 */

(Haskell)

a007775 n = a007775_list !! (n-1)

a007775_list = 1 : filter ((> 5) . a020639) [1..]

-- Reinhard Zumkeller, Jan 06 2013

(PARI) {a(n) = n\8 * 6 + 9 + 3 * (n+1)\2 * 2 - max(5, (n-2)%8) * 2} /* Michael Somos, Jun 02 2014 */

(PARI) Vec(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) + O(x^100)) \\ Altug Alkan, Nov 16 2015

CROSSREFS

Cf. A000538, A054403, A008364, A008365, A008366.

For k-rough numbers with other values of k, see A000027, A005408, A007310, A007775, A008364, A008365, A008366, A166061, A166063.

Complement is A080671.

For digital root of Fibonacci numbers indexed by this sequence, see A227896.

Sequence in context: A128974 A005776 A161850 * A070884 A135777 A090459

Adjacent sequences:  A007772 A007773 A007774 * A007776 A007777 A007778

KEYWORD

nonn,easy

AUTHOR

N. J. A. Sloane

STATUS

approved

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Last modified May 25 08:37 EDT 2017. Contains 287015 sequences.