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 A276988 a(n) is the least k such that 10*k+prime(n) is composite. 0
 1, 3, 1, 2, 1, 2, 1, 2, 1, 1, 2, 2, 1, 2, 1, 1, 1, 2, 1, 1, 2, 2, 1, 1, 2, 1, 2, 1, 1, 1, 2, 1, 1, 2, 1, 1, 2, 2, 1, 1, 1, 2, 1, 1, 1, 1, 1, 2, 1, 2, 1, 1, 2, 1, 1, 1, 1, 2, 1, 1, 2, 1, 2, 1, 1, 1, 1, 2, 1, 2, 1, 1, 1, 2, 2, 1, 1, 1, 1, 2, 1, 2, 1, 2, 2, 1, 1, 2, 1, 1 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS It appears that a(n)<3 for n>2 (checked up to 10^7). This comment is surely true since every prime except 3 equals 1 or 2 mod 3, so the addition of 10 == 1 mod 3 once or twice makes it divisible by 3. So (3 - (prime(n) mod 3)) is an upper bound. - Andrey Zabolotskiy, Nov 01 2016 LINKS Table of n, a(n) for n=1..90. EXAMPLE For n=1, 10+2=3x4 so a(1)=1; For n=2, 13 and 23 are prime, but then 30+3=3x11 so a(2)=3; For n=3, 10+5=3x5 so a(3)=1; For n=4, 17 is prime, but then 20+7=3x9 so a(4)=2. PROG (PARI) isc(n) = (n > 1) && !isprime(n); a(n) = my(k = 0, p = prime(n)); while(!isc(p+10*k), k++); k; \\ Michel Marcus, Sep 27 2016 CROSSREFS Sequence in context: A256253 A288818 A319658 * A084642 A271418 A230500 Adjacent sequences: A276985 A276986 A276987 * A276989 A276990 A276991 KEYWORD nonn AUTHOR Yves Debeuret, Sep 24 2016 EXTENSIONS More terms from Michel Marcus, Sep 27 2016 STATUS approved

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Last modified July 21 15:35 EDT 2024. Contains 374474 sequences. (Running on oeis4.)