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 A065873 Numbers k such that usigma(k) +-1 are twin primes, where usigma(k) is the sum of the unitary divisors of k (A034448). 1
 3, 5, 6, 10, 11, 17, 18, 20, 26, 29, 30, 38, 41, 44, 45, 46, 51, 55, 56, 59, 71, 80, 85, 88, 90, 98, 101, 105, 107, 114, 116, 118, 126, 132, 137, 140, 141, 145, 149, 150, 152, 153, 155, 158, 160, 161, 177, 178, 179, 185, 188, 191, 197, 203, 206, 207, 209, 212, 227 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS Amiram Eldar, Table of n, a(n) for n = 1..10000 (terms 1..1000 from Harry J. Smith) EXAMPLE 3 is a term since usigma(3) = 4 and (4-1, 4+1) = (3, 5) are twin primes. MATHEMATICA f[n_] := Block[ {a = FactorInteger[n], k = l = s = 1}, l = Length[a]; While[k <= l, s = s * (a[[k, 1]]^a[[k, 2]] + 1); k++ ]; Return[s]]; Select[ Range[250], PrimeQ[ f[ # ] + 1] && PrimeQ[ f[ # ] - 1] & ] PROG (PARI) usigma(n)= { local(f, s=1); f=factor(n); for(i=1, matsize(f)[1], s*=1 + f[i, 1]^f[i, 2]); return(s) } { n=0; for (m=1, 10^9, u=usigma(m); if (isprime(u - 1) && isprime(u + 1), write("b065873.txt", n++, " ", m); if (n==1000, return)) ) } \\ Harry J. Smith, Nov 02 2009 (MAGMA) usigma:=func; [k:k in [1..230]| IsPrime(a) and (NextPrime(a)-2 eq a) where a is usigma(k)-1]; // Marius A. Burtea, Jan 16 2020 CROSSREFS Cf. A001097, A001359, A014574, A006512, A034448. Sequence in context: A190243 A187127 A187126 * A072282 A028727 A257346 Adjacent sequences:  A065870 A065871 A065872 * A065874 A065875 A065876 KEYWORD nonn,changed AUTHOR Robert G. Wilson v, Dec 07 2001 STATUS approved

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Last modified January 26 01:48 EST 2020. Contains 331270 sequences. (Running on oeis4.)