A244435 a(n) is the smallest number m such that 2*k*m - 1 is composite for all k, 0 < k < n+1.

5, 13, 13, 62, 73, 73, 89, 118, 118, 118, 118, 118, 236, 926, 959, 959, 959, 959, 959, 959, 1063, 1474, 1474, 1474, 1667, 1667, 6118, 8249, 9098, 9098, 9098, 9098, 9098, 9098, 9098, 9098, 9098, 9098, 9098, 9098, 9098, 9098, 9098, 35573, 35573, 35573, 57448, 57448, 57448, 57448, 57448, 57448, 57448

Offset: 1

Author

Farideh Firoozbakht, Jul 19 2014

Keywords

• nonn

Comments

a(2)=a(3)=13, a(5)=a(6)=73, ... a(29)=a(30)=...=a(43)=9098, ... . A244436 gives numbers k such that a(k) is not in the set {a(k-1), a(k+1)}.

Links

• Robert Israel, Table of n, a(n) for n = 1..164

Crossrefs

Cf. A124516, A244433, A244436.

Programs

Maple

M:= 0: R:= NULL:
for m from 2 while M < 100 do
  for i from 0 while not isprime(2*i*m-1) do od:
  if i-1 > M then R:= R, m$(i-1-M); M:= i-1; fi;
od:
R; # Robert Israel, May 03 2021

PARI

isok(n, m) = for(k=1, n, my(x=2*k*m-1); if ((x==1) || isprime(x), return(0))); return (1);
a(n) = my(m=1); while(!isok(n, m), m++); m; \\ Michel Marcus, May 04 2021