## 6 Conclusions  
We demonstrated attosecond waveform synthesis using the phase-locked harmonic combs containing three and four harmonics at seeded FEL FERMI. The characterization of the generated attosecond waveforms using the technique of CoBRA confirmed the complex waveform shaping, producing attosecond pulses with a pulse duration as short as 210 \(\pm\) 5 as. The described pulse-shaping scheme with an arbitrary number \(N\) of harmonics permits the generation of pulses of sub-100 as duration. Indeed, the results reported here could promote a positive decision concerning the future upgrades of FEL facilities in this direction and possibly lead to a revision of the layout towards an optimized setup that could better fit the needs of an attosecond pulse waveform control. Furthermore, the intensities offered at FERMI, along with the described attosecond pulse synthesis, promote the development of schemes to perform nonlinear XUV experiments with subfemtosecond temporal resolution. Conceptualization, G.S. and C.C. ; machine operation, L.G., E.A. and G.D.N. ; Contribution to machine opertaion, A.A.L. ; MBES operation, R.J.S. and R.F. ; endstation preparation, M.D.F., O.P., C.C. and C.G. ; data acquisition and analysis, P.K.M., M.M. ([PERSON]), D.E., M.D.F., O.P., H.A., P.F., C.T.M., M.M. ([PERSON]), S.B., N.I., E.R.S., J.M., T.C., M.D., S.K., H.N.G., D.Y., K.U., K.C.P., C.G. and C.C. ; data interpretation, P.K.M., G.S. and C.C. ; perturbation theory, A.N.G.-G. and E.V.G. ; TDSE calculations, M.L., J.E.B. and K.J.S. ; software, P.K.M., M.R., D.Y. and G.S. ; formal analysis, P.K.M., C.C. and G.S. ; writing--original draft preparation, P.K.M. and G.S; writing--review and editing, P.K.M. and G.S. All authors have read and agreed to the published version of the manuscript. This project has received funding from the European Union's Horizon 2020 research and innovation program under the Marie Sklodowska-Curie Grant Agreement No. 641789 MEDEA. G.S. acknowledges funding from DFG, Projekt: 429805582. R.F. and R.J.S. thank the Swedish Research Council (VR) and the Knut and Alice Wallenberg Foundation for financial support. S.B., N.I., E.R.S., and J.M. thank the Swedish Research Council (VR) for support. A.A.L. acknowledges support from the U.S. Department of Energy Contract DE-AC02-76 SF00515. E.V.G. acknowledges the support of the Foundation for the Advancement of Theoretical Physics and Mathematics \"BASIS\". Research at Louisiana State University was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Contract No. DE-SC010431. Portions of this research were conducted with high-performance computing resources provided by Louisiana State University ([[http://www.hpc.lsu.edu](http://www.hpc.lsu.edu)]([http://www.hpc.lsu.edu](http://www.hpc.lsu.edu)) (accessed on 19 October 2021)) and by Louisiana Optical Network Infrastructure ([[http://hpc.loni.org](http://hpc.loni.org)]([http://hpc.loni.org](http://hpc.loni.org)) (accessed on 19 October 2021)). M.M. acknowledges support by the Deutsche Forschungsgemeinschaft (DFG (German Research Foundation))--SFB-925--Project 170620586. A.N.G. and E.V.G. acknowledge funding from the Russian Foundation for Basic reResearch (RFBR) under Project No. 20-52-12023. K.U. acknowledges support by the X-ray Free Electron Laser Utilization Research Project and the X-ray Free Electron Laser Priority Strategy Program of the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT), by the Cooperative Research Program of \"Network Joint Research Center for Materials and Devices: Dynamic Alliance for Open Innovation Bridging Human, Environment and Materials\", by the bilateral project CNR-JSPS \"Ultrafast science with extreme ultraviolet Free Electron Laser\", and by the IMRAM project for the international cooperation. G.S. acknowledges the useful discussion about the simulations and the data analysis with [PERSON] and [PERSON]. We acknowledge [PERSON], [PERSON], and [PERSON] for valuable discussions. * [PERSON] et al.