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A116518
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Odd numbers n such that n and phi(n) have the same number of divisors.
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0
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1, 3, 15, 255, 65535, 77805, 161595, 331695, 575025, 664335, 765765, 1601145, 2250885, 2380833, 2690415, 3271905, 3828825, 4107285, 5181813, 5778045, 5871285, 6007365, 6613425, 7448805, 9258795, 9787869, 9935055, 10503675
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,2
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COMMENTS
| For n<6 product of the first n Fermat primes is in the sequence because if m=2^2^n-1 and n<6 then d(m)=d(phi(m))=2^n. - Farideh Firoozbakht (mymontain(AT)yahoo.com), Aug 28 2006
(1). If p is a Sophie-Germain prime greater than 3 then m=69615*(2p+1) is in the sequence because d(m)=d(phi(m))=96. 765765, 1601145, 3271905, 4107285, 5778045, 7448805,... is the related subsequence. - Farideh Firoozbakht (mymontain(AT)yahoo.com), Aug 28 2006
(2). If p is a prime greater than 3 such that 4p+1 is prime then m=700245(4p+1) is in the sequence because d(m)=d(phi(m))=160. 20307105, 37112985, 104336505, 121142385,... is the related subsequence. - Farideh Firoozbakht (mymontain(AT)yahoo.com), Aug 28 2006
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PROG
| (PARI) forstep(n=1, 10^8, 2, if(numdiv(n)==numdiv(eulerphi(n)), print1(n, ", ")))
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CROSSREFS
| Cf. A070418.
Cf. A005384.
Sequence in context: A114735 A173146 A139289 * A050474 A051179 A122591
Adjacent sequences: A116515 A116516 A116517 * A116519 A116520 A116521
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KEYWORD
| nonn
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AUTHOR
| Max Alekseyev (maxale(AT)gmail.com), Mar 24 2006
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