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A128281 a[n] = is the least product of n odd primes m=p1*p2*...*pn, such that (d+m/d)/2 are all primes for each d dividing m. 4
3, 21, 105, 1365, 884037 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

Table of n, a(n) for n=1..5.

EXAMPLE

105=3.5.7, (3.5.7+1)/2=53, (3+5.7)/2=19, (5+3.7)/2=13, (7+3.5)/2=11 are all primes and 105 is the least such number which is the product of 3 primes, so a[3]=3.

CROSSREFS

Cf. A128276, A128282, A128283, A128284, A128285, A128286.

Sequence in context: A306093 A076207 A134057 * A034268 A140451 A054147

Adjacent sequences:  A128278 A128279 A128280 * A128282 A128283 A128284

KEYWORD

hard,more,nonn

AUTHOR

Kok Seng Chua (chuakokseng(AT)hotmail.com), Mar 05 2007

STATUS

approved

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Last modified November 19 00:12 EST 2019. Contains 329310 sequences. (Running on oeis4.)