are available for example in connection with familiar function spaces. The 1970s experienced a real boom of this theory involving weighted Lebesgue spaces and fundamental papers were written ([@Muc:72; @Sa:82] for the Hardy–Littlewood maximal operator, [@MW:71] for singular and fractional integrals, [@CF:74; @HMW; @MW:76] for the Hilbert transform). Later it became apparent that Lebesgue spaces are not sufficient for describing all the important situations and other function spaces were investigated. Classical Lorentz spaces which originated in the 1950s and have been occurring occasionally later (see [@Bag:83; @Boy:67]) became extremely fashionable in the 1990s when the fundamental papers [@AM:90; @Sa:90] appeared. Various important and deep results were obtained, see for example [@ACS:12; @CO:15; @CS:93; @CS:97; @CE:97]. Orlicz spaces which generalize Lebesgue’s scale in a direction essentially different from Lorentz spaces, received much attention too, see for instance [@BK:84; @BP:87; @Cia:97; @Cia:99; @CM:19; @Gal:88; @Mus:19]. The results naturally found their way into important monographs that are considered classic these days, see [@CMP:11; @DHHR:11; @GR:85; @Mabook; @Ruz:00; @Ste:70; @Ste:93; @SW:71]. Let us point out that, in particular, in the monograph [@Mabook], among plenty of other fundamental results, the significance of the connection between embeddings and integral operators is explained in great detail.

On 