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A211767 Lesser of twin primes p, p+2 with unique values of sigma(p) and sigma(p+2); sigma(n) = A000203(n) = sum of divisors of n. 4
3, 5, 197, 281, 347, 461, 641, 821, 857, 1289, 1697, 1721, 1787, 1877, 2081, 2141, 2381, 2549, 2801, 3257, 3539, 3557, 3929, 4019, 4241, 4637, 4721, 5441, 5477, 5501, 5657, 6449, 6689, 6701, 6761, 6827, 6947, 7457, 7589, 7877, 8009, 8387, 8537, 8597, 8627 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Subsequence of A211656, A211660, A211678.

LINKS

Amiram Eldar, Table of n, a(n) for n = 1..10000

EXAMPLE

Prime 197 is in sequence because 197 and 199 are twin primes, sigma(197) = 198, sigma(199) = 200 and there are no other numbers m, n with sigma(m) = 198 or sigma(n) = 200.

MATHEMATICA

d = DivisorSigma[1, Range[10000]]; t = Transpose[Select[Tally[Sort[d]], #[[2]] == 1 && #[[1]] <= Length[d] &]][[1]]; t2 = Sort[Flatten[Table[Position[d, i], {i, t}]]]; t3 = Select[t2, PrimeQ]; tp = {}; Do[If[t3[[i+1]] - t3[[i]] == 2, AppendTo[tp, t3[[i]]]], {i, Length[t3] - 1}]; tp (* T. D. Noe, Apr 26 2012 *)

CROSSREFS

Cf. A211678 (twin primes p, p+2 with unique values of sigma(p) and sigma(p+2)), A211769 (greater of twin primes p, p+2 with unique values of sigma(p) and sigma(p+2)).

Sequence in context: A337158 A333413 A104996 * A224537 A175133 A012817

Adjacent sequences:  A211764 A211765 A211766 * A211768 A211769 A211770

KEYWORD

nonn

AUTHOR

Jaroslav Krizek, Apr 20 2012

EXTENSIONS

A-number corrected by Jaroslav Krizek, Mar 17 2013

STATUS

approved

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Last modified May 23 00:47 EDT 2022. Contains 353959 sequences. (Running on oeis4.)