This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A115948 #13 Jul 20 2014 00:19:28 %S A115948 8,32,13,12,156,184,319,464,341,496,301,308,9,952,472,508,1191,922, %T A115948 2359,688,1800,2668,2291,3109,2888,4860,412,4691,604,2875,4523,2236, %U A115948 3856,5659,2016,8662,3259,8852,13239,6953,1344,6277,7357,2857,11660,18193 %N A115948 a(n) = (2^(semiprime(n)-1)) modulo (semiprime(n)^2). %C A115948 Wieferich function of semiprimes. %C A115948 This appears in the search for the semiprime analogy to A001220 Wieferich primes p: p^2 divides 2^(p-1) - 1. That is, the Wieferich function W(p) of primes p is W(p) = 2^(p-1) modulo p^2 and a (rare!) Wieferich prime (A001220) is one such that W(p) = 1. The current sequence is W(semiprime(n)). Any semiprime s for which W(s) = 1 would be a "Wieferich semiprime." This is also related to Fermat's "little theorem" that for any odd prime p we have 2^(p-1) == 1 modulo p. %C A115948 Such a "Wieferich semiprime" would be a special case of a "Wieferich pseudoprime", i.e. it would be a composite integer that is one more than a term in A240719 and has two prime factors. - _Felix Fröhlich_, Jul 16 2014 %D A115948 R. Crandall and C. Pomerance, Prime Numbers: A Computational Perspective, Springer, NY, 2001; see p. 28. %D A115948 R. K. Guy, Unsolved Problems in Number Theory, A3. %D A115948 G. H. Hardy and E. M. Wright, An Introduction to the Theory of Numbers, 5th ed., Oxford Univ. Press, 1979, th. 91. %F A115948 a(n) = (2^(A001358(n)-1)) modulo (A001358(n)^2). %t A115948 PowerMod[2, # - 1, #^2] & /@ Select[ Range@141, Plus @@ Last /@ FactorInteger@# == 2 &] (* _Robert G. Wilson v_ *) %Y A115948 Cf. A001220, A001358. %K A115948 easy,nonn %O A115948 1,1 %A A115948 _Jonathan Vos Post_, Mar 14 2006 %E A115948 More terms from _Robert G. Wilson v_, Mar 14 2006