

A353203


Let b be a composite number, c be the smallest composite number greater than b and coprime to b, and d = cb. This sequence contains all b such that d is neither a prime nor a square.


1



67613590, 72808450, 125918320, 153469030, 190281850, 229119880, 328315900, 339204910, 360203140, 395961280, 447304000, 450075340, 692309530, 844334920, 861327610, 909001390, 1029358330, 1166831380, 1178236510, 1321005400, 1344348610, 1366379080, 2035500610, 2045710810, 2156564410
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OFFSET

1,1


COMMENTS

Other such terms are 18806843674476 and 18806855958880.
a(n) is even. Proof: If a(n) = b is odd then c = a(n) + 1 where gcd(b, c) = 1 and d = cb = 1 which is a square. Contradiction.  David A. Corneth, May 01 2022


LINKS



EXAMPLE

If b = 6, then c = 25 and d = cb = 19 (prime), so 6 is not in the sequence.
If b = 67613590, then c = 67613611, and d = cb = 21 (neither prime nor square), so 67613590 is in the sequence.


MATHEMATICA

c[n_]:=Module[{k=n+1}, While[GCD[n, k]!=1PrimeQ[k], k++]; k];
Select[Range[10^8], CompositeQ[#]&&CompositeQ[c[#]#]&&!IntegerQ[Sqrt[c[#]#]]&]


CROSSREFS



KEYWORD

nonn


AUTHOR



STATUS

approved



