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A339500
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Maximum length of sequences of prime numbers that start with prime(n) and are arithmetic progressions of common difference less than prime(n).
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2
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2, 3, 2, 3, 4, 2, 4, 3, 3, 3, 3, 5, 4, 4, 4, 4, 4, 4, 4, 5, 3, 4, 4, 3, 4, 4, 5, 6, 3, 5, 4, 4, 5, 3, 4, 5, 6, 4, 4, 4, 4, 4, 4, 4, 4, 3, 3, 5, 4, 3, 4, 6, 4, 4, 6, 5, 4, 4, 5, 6, 4, 4, 5, 4, 4, 4, 6, 4, 4, 4, 5, 6, 5, 4, 5, 5, 6, 6, 5, 9, 5, 4
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OFFSET
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1,1
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LINKS
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EXAMPLE
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a(4) = 3 since 7 is the fourth prime number and 7, 13 and 19 are primes in arithmetic progression of common difference equal to 6 and there is no longer arithmetic sequence starting with 7 of common difference < 7 which contains only prime numbers.
a(12) = 5 since prime(12) = 37 and 67, 97, 127 and 157 are also primes in arithmetic progression of common difference 30 < 37.
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PROG
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my(p=prime(n), bp, bk);
forprime(np=p+1, 2*p, for(k=2, +oo, if(!isprime(p+k*(np-p)), if(k>bk, bk=k; bp=np; ); break); ); );
return(bk);
}
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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STATUS
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approved
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