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A080611 a(n) is the smallest number m >= 2 for which the set of prime factors of m, m-1 and m+1 contains at least the first n primes. 0
2, 2, 4, 6, 21, 155, 441, 2925, 10165, 342056, 2781505, 10631544, 163886800, 498936010, 5163068911, 794010643700, 17635639237580, 353823355745574, 16828233620277430, 224220167903546529, 11990471619719586785, 113367767003198032480, 4446177962278202834685, 118332081735203144063619, 1103720538399012083835935, 78239926422758111576984420 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

This sequence is of use in non-decimal systems whereby digits in base a(n) can be tested using simple addition tricks [and no higher operations] to determine if the number represented is relatively prime with respect to the first n primes.

The addition trick for base a(n) is to sum digits to do a(n)-1 divisibility tests and alternately add and subtract digits to perform the a(n)+1 test. In base 10 we add digits to find 9-divisibility or add-subtract digits (e.g. 132 = 2-3+1 = 0 is divisible by 11) to find divisibility by 11.

a(5) = 21 because 20, 21 and 22 have between them all 5 prime factors 2,3,5,7,11. - Don Reble (djr(AT)nk.ca), Feb 27 2003

LINKS

Table of n, a(n) for n=1..26.

Jeffrey C. Jacobs, Time Horse Home.

Robert Munafo, Interesting Numbers.

FORMULA

a(n) is the smallest number such that the product [a(n)-1]a(n)[a(n)+1] has prime factors which include the first n ordinal primes excluding 1 (see A033946).

EXAMPLE

a(1) = 1 since we assume 0 and 1 have no nontrivial prime factors, thus a(1)+1 is the only term with factors, {2}.

a(4) = 6 because a(4)-1 = 5, thus the set of prime factors {5}; a(4) = 2*3, thus the set of prime factors {2, 3} and a(4)+1 = 7 with the set of prime factors {7}. The combined set, {2, 3, 5, 7} contains the first 4 prime numbers (not including 1) and because there are no numbers less than 6 with this property, a(4) = 6.

PROG

a = 1 firstNPrimes = getFirstNPrimes(n) /* A033946[1:n+1] */ a = 0 do { ++a; primes = prime_factors(a) primes.union(prime_factors(a-1)) primes.union(prime_factors(a+1)) } while (!(firstNPrimes is_subset_of primes)) return a;

CROSSREFS

Cf. A033946.

Sequence in context: A134041 A069925 A227315 * A171421 A072707 A086105

Adjacent sequences:  A080608 A080609 A080610 * A080612 A080613 A080614

KEYWORD

nonn

AUTHOR

Jeffrey C. Jacobs (darklord(AT)timehorse.com), Feb 26 2003

EXTENSIONS

More terms from Don Reble (djr(AT)nk.ca), Feb 27 2003

STATUS

approved

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Last modified April 30 13:46 EDT 2017. Contains 285669 sequences.