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Search: id:A079153
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| A079153 |
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Primes p such that both p-1 and p+1 have at most 3 prime factors, counted with multiplicity; i.e. primes p such that bigomega(p-1) <= 3 and bigomega(p+1) <= 3, where bigomega(n) = A001222(n). |
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+0
4
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| 2, 3, 5, 7, 11, 13, 19, 29, 43, 67, 173, 283, 317, 653, 787, 907, 1867, 2083, 2693, 2803, 3413, 3643, 3677, 4253, 4363, 4723, 5443, 5717, 6197, 6547, 6653, 8563, 8573, 9067, 9187, 9403, 9643, 10733, 11443, 11587, 12163, 12917, 13997, 14107, 14683, 15187
(list; graph; listen)
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OFFSET
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1,1
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COMMENT
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Sum of reciprocals ~ 1.495. There are 3528 primes of this kind <= 10^7.
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EXAMPLE
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907 is in the sequence because both 907-1 = 2*3*151 and 907+1 = 2*2*227 have 3 prime factors.
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PROGRAM
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(PARI) s(n) = {sr=0; ct=0; forprime(x=2, n, if(bigomega(x-1) < 4 && bigomega(x+1) < 4, print1(x" "); sr+=1.0/x; ct+=1; ); ); print(); print(ct" "sr); } \\ Lists primes p<=n such that both p-1 and p+1 have at most 3 prime factors.
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CROSSREFS
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Intersection of A079150 and A079151. Cf. A079152.
Adjacent sequences: A079150 A079151 A079152 this_sequence A079154 A079155 A079156
Sequence in context: A020588 A114111 A113188 this_sequence A020616 A067910 A075430
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KEYWORD
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easy,nonn
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AUTHOR
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Cino Hilliard (hillcino368(AT)gmail.com), Dec 27 2002
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