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A077425 a(n) == 1 (mod 4) (see A016813), but not a square (i.e., not in A000290). 14
5, 13, 17, 21, 29, 33, 37, 41, 45, 53, 57, 61, 65, 69, 73, 77, 85, 89, 93, 97, 101, 105, 109, 113, 117, 125, 129, 133, 137, 141, 145, 149, 153, 157, 161, 165, 173, 177, 181, 185, 189, 193, 197, 201, 205, 209, 213, 217, 221, 229, 233, 237, 241, 245, 249, 253, 257 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
COMMENTS
The Pell equation x^2 - a(n)*y^2 = +4 has infinitely many (integer) solutions (see A077428 and A078355).
These are the odd numbers in A079896. The even ones are 4*A000037. - Wolfdieter Lang, Sep 15 2015
First differences: 8, 4, 4, 8, 4, 4, 4, 4, 8, 4, 4, 4, 4, 4, 4, 8, ... , only 4's and 8's?. - Paul Curtz, Apr 11 2019
Yes. There are only 4's and 8's. Proof: Only multiples of 4 may appear. The 4's correspond to successive composite in A016813, whereas an 8 corresponds to a square. A greater multiple of 4 would imply to have at least 2 consecutive squares in A016813, which is not possible since 2 consecutive squares cannot have a difference of 4. That sequence of 4's and 8's can be obtained with A010052 (without the 1st term) where the 0's are replaced with 4's and 1's replaced with 8's. - Michel Marcus, Apr 16 2019
LINKS
S. R. Finch, Class number theory [Cached copy, with permission of the author]
MAPLE
A077425 := proc(n::integer) local resul, i ; resul := 5 ; i := 1 ; while i < n do resul := resul+4 ; while issqr(resul) do resul := resul+4 ; od ; i:= i+1 ; od ; RETURN(resul) ; end proc:
seq(A077425(n), n=1..31) ; # R. J. Mathar, Apr 25 2006
MATHEMATICA
Select[Range[5, 300, 4], !IntegerQ[Sqrt[#]]&] (* Harvey P. Dale, Dec 05 2012 *)
PROG
(PARI) [n | n <- vector(100, n, 4*n+1), !issquare(n)] \\ Charles R Greathouse IV, Mar 11 2014
(PARI) list(lim)=my(v=List()); for(s=2, sqrtint((lim\=1)+1), forstep(n=s^2 + if(s%2, 4, 1), min((s+1)^2-1, lim), 4, listput(v, n))); Vec(v) \\ Charles R Greathouse IV, Nov 04 2021
CROSSREFS
Intersection of A016813 and A000037.
Sequence in context: A174361 A226165 A166409 * A039955 A213340 A014539
KEYWORD
nonn,easy
AUTHOR
Wolfdieter Lang, Nov 29 2002
EXTENSIONS
More terms from Max Alekseyev, Mar 03 2010
STATUS
approved

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Last modified April 21 14:18 EDT 2024. Contains 371870 sequences. (Running on oeis4.)