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A047978 Let (p1,p2), (p3,p4) be pairs of twin primes with p1*p2=p3+p4-1; sequence gives values of p3. 3
17, 71, 881, 2591, 179999, 206081, 388961, 2268449, 2836961, 3612671, 6329681, 6415361, 10342151, 18800711, 28486151, 32080049, 40878881, 89753201, 97050311, 127552391, 154844801, 183169799, 230953031, 344531249 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
LINKS
Charles R Greathouse IV, Table of n, a(n) for n = 1..10000
EXAMPLE
E.g. 5*7=17+19-1,11*13=71+73-1.
MATHEMATICA
Select[(Times@@#-1)/2&/@Select[Partition[Prime[Range[3000]], 2, 1], #[[2]]-#[[1]]==2&], AllTrue[ #+{0, 2}, PrimeQ]&] (* Harvey P. Dale, May 19 2023 *)
PROG
(PARI) list(lim)=my(v=List(), p=3, r); forprime(q=5, sqrtint(lim\1*2+2)+1, if(q-p==2 && isprime(r=p*q\2) && isprime(r+2), listput(v, r)); p=q); Vec(v) \\ Charles R Greathouse IV, Sep 29 2015
(PARI) is(n)=my(p=sqrtint(2*n+1)); p*(p+2)==2*n+1 && isprime(n) && isprime(n+2) && isprime(p) && isprime(p+2) \\ Charles R Greathouse IV, Sep 29 2015
CROSSREFS
Sequence in context: A320895 A288748 A069496 * A050524 A214530 A087514
KEYWORD
nonn
AUTHOR
STATUS
approved

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Last modified June 21 18:40 EDT 2024. Contains 373556 sequences. (Running on oeis4.)