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A307217 Semiprimes p*q such that 2^(p+q) == 1 (mod p*q). 0
9, 15, 35, 119, 5543, 74447, 90859, 110767, 222179, 389993, 1526849, 2927297, 3626699, 4559939, 24017531, 137051711, 160832099, 229731743, 627699239, 880021141, 1001124539, 1041287603, 1104903617, 1592658611, 1717999139, 8843679683, 15575602979, 15614760199, 20374337479 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

For k > 9, these are semiprimes k such that 2^(k+1) == 1 (mod k): semiprimes in A187787.

In this sequence, only 9 is a perfect square. - Jinyuan Wang, Mar 30 2019

LINKS

Table of n, a(n) for n=1..29.

PROG

(PARI) isok(k) = (bigomega(k)==2) && (Mod(2, k)^(k+1) == 1); \\ (for k > 9) Michel Marcus, Mar 29 2019

(Perl) use ntheory ":all"; forsemiprimes { print "$_\n" if powmod(2, vecsum(factor($_)), $_) == 1 } 4, 1e7; # Daniel Suteu, Mar 30 2019

CROSSREFS

Cf. A001358, A046315, A127104, A187787, A208728.

Sequence in context: A058505 A133763 A146475 * A100556 A057478 A128687

Adjacent sequences:  A307214 A307215 A307216 * A307218 A307219 A307220

KEYWORD

nonn

AUTHOR

Thomas Ordowski, Mar 29 2019

EXTENSIONS

a(7)-a(18) from Amiram Eldar, Mar 29 2019

a(19)-a(29) from Daniel Suteu, Mar 29 2019

STATUS

approved

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Last modified June 15 15:16 EDT 2021. Contains 345049 sequences. (Running on oeis4.)