# Singular cardinals and strong extenders

• A. W. Apter, J. Cummings, and J. D. Hamkins, “Singular cardinals and strong extenders,” Central European Journal of Mathematics, vol. 11, iss. 9, pp. 1628-1634, 2013.
@article {ApterCummingsHamkins2013:SingularCardinalsAndStrongExtenders,
AUTHOR = {Apter, Arthur W. and Cummings, James and Hamkins, Joel David},
TITLE = {Singular cardinals and strong extenders},
JOURNAL = {Central European Journal of Mathematics},
VOLUME = {11},
YEAR = {2013},
NUMBER = {9},
PAGES = {1628--1634},
ISSN = {1895-1074},
MRCLASS = {03E55 (03E35 03E45)},
MRNUMBER = {3071929},
MRREVIEWER = {Samuel Gomes da Silva},
DOI = {10.2478/s11533-013-0265-1},
URL = {http://jdh.hamkins.org/singular-cardinals-strong-extenders/},
eprint = {1206.3703},
archivePrefix = {arXiv},
primaryClass = {math.LO},
}

Brent Cody asked the question whether the situation can arise that one has an elementary embedding $j:V\to M$ witnessing the $\theta$-strongness of a cardinal $\kappa$, but where $\theta$ is regular in $M$ and singular in $V$.

In this article, we investigate the various circumstances in which this does and does not happen, the circumstances under which there exist a singular cardinal $\mu$ and a short $(\kappa, \mu)$-extender $E$ witnessing “$\kappa$ is $\mu$-strong”, such that $\mu$ is singular in $Ult(V, E)$.